Add the explicitly mutable flag for creating a PendingIntent

- Android S+ to specify explicitly either FLAG_MUTABLE or FLAG_IMMUTABLE
  when creating a PendingIntent.
- Suggest to use the FLAG_IMMUTABLE as default. Change it to
  FLAG_MUTABLE while errors occur.

Fixes: 170162598
Fixes: 170162878
Fixes: 170163592
Fixes: 170164316
Fixes: 170164458
Fixes: 170164327
Fixes: 169794524
Fixes: 170165106
Test: build pass, manual test add an account and volume control panel
Change-Id: I5aec028f82bd74fc4530b95a7c144811055ae2c5