Update removeAll path

1) Make the resulting face/fingerprint can be @Nullable. Not all
   removal callers care about the template that was removed, just
   that they were all removed.

2) FingerprintService/FaceService should wait for all providers to
   finish removing before notifying the upper layer of completion

Fixes: 183758063
Test: atest CtsBiometricsTestCases, notice
      testLockoutResetRequestedAfterCredentialUnlock finishes
      10 seconds quicker now (latch in LSS doesn't time out anymore)
Change-Id: I87c50ccf5524fd5440c9a6c2c7a0fd4cc61e684d