AnrTimer errors are now managed in a RingBuffer

This addresses comments from the previous CL:
 * AnrTimer is no longer an abstract class
 * The error list now uses a RingBuffer
 * The unused parameter 'when' in onExpired() is removed
 * A few variables/parameters are renamed.

Test: atest
 * FrameworksServicesTests:com.android.server.am
 * FrameworksMockingServicesTests:com.android.server.am
 * CtsAppTestCases
Bug: 282428924

Change-Id: Id3952003b0b31d845712cab07f3d796f39423f21