Clean up biometric AIDL binderDied handling

1) We should only register one death recipient - in the top-level
   provider class. The provider can notify all sensors to clean up
   when the HAL dies
2) Always set mCurrentSession to null. Otherwise, if current client
   is null, or non-interruptable, subsequent attempts to access the
   HAL will result in a DeadObjectException

Fixes: 179097844
Test: atest com.android.server.biometrics
Change-Id: I8604dc1988b8b7f3572596b1431da4d6e5c393c2