Fix the overrides file corruption due to shutdown

The dynamic overrides file could be corrupted by shutdown during
writes. This CL introduces the backup file to avoid corruption.

Test: manual
   1. push a backup file to /data/misc/appcompat/
   2. push a corrupted file to /data/misc/appcompat/
   3. Reboot and see if it reads the backup file
Bug: 196909332
Change-Id: If13547f04a4712ab18dcb97ffee698327797f97b