Access mCurrentOperation in main handler

BiometricScheduler::mCurrentOperation may be accessed on binder thread
from FingerprintProvider, and it causes NullPointerException when
mCurrentOperation was free on other thread.

Use Consumer<> interface to return BaseClientMonitor asynchronously on
BiometricScheduler's main handler.

Bug: 228396526
Test: atest BiometricSchedulerTest FingerprintProviderTest \
      Fingerprint21Test
Test: Manually test UDFPS unlock on device
Change-Id: Iea91f21879fc097932b9d2d08c5d60a99a4f6648