Lollipop and above - How do I install an APK as a system app?

Question

Back in the day, installing an app as a system app was as simple as moving the APK file to /system/app or /system/priv-app. Removing them was as simple as deleting the APK.

Now, while I'm customizing the phone's stock ROM, I've noticed that each and every app on the system partition was inside their own folder. I've tried to simply drop the APK in /system/app or /system/priv-app, but they don't work.

How do I install an app as a system app on Lollipop? I suspect something's up with SELinux and context, but I don't have root access. However, I have read/write access to the filesystem (by mounting the image in Linux).

UPDATE: I've tried Death Mask Salesman's suggestion and taken a look at logcat. The apps I want to pre-install (e.g. Instagram) are throwing this error:

01-01 01:04:45.108 5538-5538/? E/AndroidRuntime: FATAL EXCEPTION: main


 Process: com.instagram.android, PID: 5538


 java.lang.RuntimeException: Unable to instantiate application com.instagram.app.InstagramAppShell: java.lang.ClassNotFoundException: Didn't find class "com.instagram.app.InstagramAppShell" on path: DexPathList[[zip file "/system/framework/com.google.android.maps.jar"],nativeLibraryDirectories=[/custpack/app/removeable/withlibs/com.instagram.android-1/lib/arm, /vendor/lib, /system/lib]]


  at android.app.LoadedApk.makeApplication(LoadedApk.java:572)


  at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4818)


  at android.app.ActivityThread.access$1500(ActivityThread.java:178)


  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1531)


  at android.os.Handler.dispatchMessage(Handler.java:111)


  at android.os.Looper.loop(Looper.java:194)


  at android.app.ActivityThread.main(ActivityThread.java:5624)


  at java.lang.reflect.Method.invoke(Native Method)


  at java.lang.reflect.Method.invoke(Method.java:372)


  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:959)


  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:754)


  Caused by: java.lang.ClassNotFoundException: Didn't find class "com.instagram.app.InstagramAppShell" on path: DexPathList[[zip file "/system/framework/com.google.android.maps.jar"],nativeLibraryDirectories=[/custpack/app/removeable/withlibs/com.instagram.android-1/lib/arm, /vendor/lib, /system/lib]]


  at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:56)


  at java.lang.ClassLoader.loadClass(ClassLoader.java:511)


  at java.lang.ClassLoader.loadClass(ClassLoader.java:469)


  at android.app.Instrumentation.newApplication(Instrumentation.java:985)


  at android.app.LoadedApk.makeApplication(LoadedApk.java:567)


  at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4818)Â 


  at android.app.ActivityThread.access$1500(ActivityThread.java:178)Â 


  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1531)Â 


  at android.os.Handler.dispatchMessage(Handler.java:111)Â 


  at android.os.Looper.loop(Looper.java:194)Â 


  at android.app.ActivityThread.main(ActivityThread.java:5624)Â 


  at java.lang.reflect.Method.invoke(Native Method)Â 


  at java.lang.reflect.Method.invoke(Method.java:372)Â 


  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:959)Â 


  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:754)Â 


 Suppressed: java.lang.ClassNotFoundException: com.instagram.app.InstagramAppShell


  at java.lang.Class.classForName(Native Method)


  at java.lang.BootClassLoader.findClass(ClassLoader.java:781)


  at java.lang.BootClassLoader.loadClass(ClassLoader.java:841)


  at java.lang.ClassLoader.loadClass(ClassLoader.java:504)


... 13 more


  Caused by: java.lang.NoClassDefFoundError: Class not found using the boot class loader; no stack available

Symptoms:

All of the apps I've installed force closes

Their package names and the generic logo shows up on the launcher instead of their app name and logo

They simply don't work

To people planning to flag this as a duplicate, these answers don't work/are outdated/not applicable (not a live system/no root access):

How do I properly install a system app given its .apk?

Convert to system app by manually moving the APK to "system/app" folder in Lollipop

How to Install App as System App

How to install system app on Lollipop as su doesn't exist

Installing system apps on Android 4.4

Add system apps in a custom rom

How do I install an app with system-rights on a non-rooted smartphone?

Installation in system/app without root

How to Install system app with SE Linux enforced on Galaxy s7 Android 6.0.1

— Aloha | Sources

Answer

Okay, I got it working. Here's how I did it:

Copy the APK file to /system/app/some-folder/ or /system/priv-app/xyz-folder/.
- Keeping them in their own folders may/will prevent conflicts. Some apps want to have their libs and other files extracted and placed alongside the APK in specific folders, although they're pretty rare.
- Try checking out some folders of other system apps. Notice that some of them have a lib or other folder beside the APK.
- That's why almost all of the apps in the folders mentioned above have their APKs inside their own folders.
- Following step 1 is usually enough.

Change the APK file's context using the command
- chcon u:object_r:system_file:s0 /path/to/apk-file.apk
- Or, if you copied it into its own folder, run chcon -R u:object_r:system_file:s0 /folder/where/the/APK/is/saved.

If the context isn't changed, Android won't treat your app as a system app. It will show up on the launcher as an app with a generic icon and zzz.package.name as the name.

In ye olde days (e.g. Gingerbread), you simply had to copy the APK in /system/app/ and set proper permissions. Times have changed.

— Aloha | Sources

Lollipop and above - How do I install an APK as a system app?

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