Polishing for migration to disable Weaver

- Prevent device from ending up in bad state if it rolls back to old
  build after the migration but before the boot has completed.
- Place a (generous) maximum number of retries on unwrapping the SP.
- Improve a comment.

Bug: 368151356
Test: atest FrameworksServicesTests:com.android.server.locksettings
Flag: EXEMPT uses config option instead
Change-Id: If05d11a0a884294896223aedd79d7f689380dc58