Provide injectable clock for AnrTimerService

This defines a test mode version of the abstract Clock used by the
native AnrTimerService.  The abstract Clock offers a method to set the
current time; this method does nothing for the production Clock but
adjusts the current time for the test mode clock.

Test timers rely on test code to manually advance the clock's view of
current time.  Test clocks are allocated one per AnrTimerService and
are destroyed along with the AnrTimerService.  This gives better test
isolation.

AnrTimer instances can be created in test mode; these instances use a
unique test mode clock.  Methods to set the time of a test mode clock
are plumbed up to AnrTimer.  Again, the methods do nothing unless the
AnrTimer is in test mode.

Note that production AnrTimers use a singleton clock based on the OS
clock and timerfd routes, while test-mode AnrTimers each have their
own test-mode clodk.

Unit tests have been updated to use the test mode for some tests.
Some tests still rely on production timers to verify proper behavior
when a production clock is being used.

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

Change-Id: I165e949db735f1e90db0dfbad094998bb676d4dd