2/n: Clean up ClientMonitor

This is the first step in moving ClientMonitors and their
implementations entirely to their own files.

Remove as many abstract methods in ClientMonitor and its subclass as
possible. These classes should be decoupled from their owners as much
as possible. This should make it much easier to support future extensions.

Bug: 157790417
Test: Enroll/Auth on fingerprint/face devices

TODO: Test more thoroughly

Change-Id: Idfa421ccad93e1940068f35eb74a1b4932f44de6