Iterate on storage permissions model.

This change updates the permissions design to use app-ops for
controlling write access, which is only extended to the default app
for a particular collection type.

Bug: 119713234
Test: atest android.appsecurity.cts.PermissionsHostTest
Test: atest android.appsecurity.cts.ExternalStorageHostTest
Test: atest cts/tests/tests/provider/src/android/provider/cts/MediaStore*
Change-Id: I40811ff175b3b8410b58ed901948a23a56f8a8c2