How to make default application in android?

The term you mentioned 'Default app' means i assume you want to make your app as 'System app'. If your phone is rooted successfully you can do this with numerous methods.

1. File Explorer method
   If you have a file explorer with root permission granted (ex. ES File explorer etc), it is very easy.

   
   
   
   
   

   • Just obtain an 'APK' file of your app.

   
   

   • Copy the APK file. Navigate to '/system/app'. paste it under '/system/app' folder.

   
   

   • Long press 'APK' to change its properties. Under permissions property, click on change and set it to rw- r-- r--. ( Thats user: read and write, group: read only and other: read only. Thats it! Reboot your device.

   
   



2. ADB method
   Same can be done with the help of ADB. You just have to copy and paste your Apps APK file under '/system/app' folder.

   
   
   
   
   

   • To do this, connect your device to your pc and make sure you have ADB drivers are installed correctly. Also enable the usb debugging.

   
   

   • Open command prompt and type:

   
   

   • adb shell

   
   

   • su

   
   

   • cd /data/app

   
   

   • mv 'your_app name' /system/app/'your_app_name'. Replace 'your_app_name' with obviously name of your application without quotes.

   
   

   • Again exit shell and reboot.

   
   



3. Apps method
   This is most simple method. There are many apps available to do this task. One popular app is 'Titanium Backup'. But as per my knowledge 'convert to system app' feature is paid version only. So i prefer one of above method only.

Tip: do the tricks only if you know what you are doing and you are familiar with android file system at your own risk.