6/n: Rename ClientMonitor to BaseClientMonitor

This CL is purely a rename-only CL.

Next CL will add something like HalClientMonitor, which extends
BaseClientMonitor. This will allow certain operations that are not
HAL-dependent (e.g. InvalidationRequesterClient) to extend from
BaseClientMonitor, which will not require HAL templating.

Bug: 159667191
Test: atest com.android.server.biometrics
Change-Id: I281c599b484083f343fd2728e0ea48539cd57063