Abstract clock functions in AnrTimerService

This abstracts the notion of "clock", as used by the native
AnrTimerService.  A Clock object has attributes of current time and
alarm time and a method to wait until the current time is greater than
or equal to the alarm time.

This is the groundwork for creating ain injectable clock for testing.

Production timers use a singleton clock that is based on the OS clock
and timerfd routimes.

There is no behavioral change in this CL.

Flag: EXEMPT refactor
Bug: 407584606
Test: atest
 * FrameworksServicesTests:AnrTimerTest
 * FrameworksServicesTests:com.android.server.am
 * FrameworksMockingServicesTests:com.android.server.am
Change-Id: Ia150b776325ad596ba55b8af738e4383b38316af