Address AnrTimer review comments

This CL address three review comments from an earlier commit:
 1. The call to register_android_server_utils_AnrTimer() is now
    sorted, per the style guide.
 2. An iterator error is fixed in AnrTimerService::Ticker::remove().
 3. NativeAllocationRegistry is used in lieu of finalizers in
    AnrTimer.

Ran AnrTimerTest.testGarbageCollection manually, in addition to the
automated tests.

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

Bug: 282428924
Change-Id: I8cbbec2e74cd101685028958e450130678e4fdef