Initial ANR timer service

Bug: 282428924

Create a service that runs ANR timers in the background.  The service
can collect statistics across different ANRs.  The current commit
provides a legacy service, based on message handlers; this is how
existing ANR timers are handled.  However, the commit lays the
groundwork for a native runtime timer implementation.

Test: atest
 * FrameworksServicesTests:com.android.server.am
 * FrameworksMockingServicesTests:com.android.server.am
 * CtsAppTestCases

Change-Id: I33b65d1a6c3348a5704191726e704743c71d5c4c