Move AnrTimer to com.android.server.util

Move the new AnrTimer class from services.am to the android server
utility package. This moves code around but changes no functionality
except that the unit test is simplified.  The simplification is in
preparation for a complete reworking of the feature code.

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

Bug: 282428924

Change-Id: Ic1cdf6832e6ddd04d1bfc0e8883ae3aed9c53262