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.

Bug: 172415655
Test: build pass, manual test to start/remove discoverable timeout
Change-Id: I94e26c44eb8aebb8d9a67b9d29e6bbac768a40e7