Make native ANR timers opt-out

New AnrTimer clients can choose between legacy MessageQueue
implementations and the full AnrTimer service.  Previously, this was
done globally through a single trunk-stable flag.  Now, individual
clients can switch based on their own trunk-stable flags.

Flag: EXEMPT refactor
Bug: 408440679
Test: atest
 * FrameworksServicesTests:AnrTimerTest
 * FrameworksMockingServicesTests_com_android_server_job
Change-Id: Ib1cf7cfe49a48745d5bf34677b693c7936456114