Issue scheduleWatchdog command on background thread

FaceManager/FingerprintManager#scheduleWatchdog commands do Binder IPC
in fire-and-forget manner. Move them to background thread to avoid
binder jank.

Bug: 279928682
Test: DeviceEntryFaceAuthRepository is covered by passing unit tests.
      Tested face auth change on a cheetah.
Change-Id: I8a0e6578dc871cbb5e08aef9250d174f79c4f765