Refactor and test AnrTimer early notifications

This change updates how AnrTimerService handles early notifications
and tracing.  Tracing is modified to use the SplitPoint class that was
introduced to support early notifications.

AnrTimer allows creation of "raw" SplitPoints.  Unit tests have been
written to test all of this.

Flag: EXEMPT test-only
Bug: 407584606
Test: atest
 * FrameworksServicesTests:AnrTimerTest
 * FrameworksServicesTests:com.android.server.am
 * FrameworksMockingServicesTests:com.android.server.am
Change-Id: Ib925cb3c002864a92c13af1a16811032c68b6784