Minor cleanup of AnrTimerTest

Address some review comments from a previous CL.  There is no change
in behavior.

Flag: EXEMPT test-only
Bug: 407584606
Test: atest
 * FrameworksServicesTests:AnrTimerTest
 * FrameworksServicesTests:com.android.server.am
 * FrameworksMockingServicesTests:com.android.server.am
Change-Id: Ie3adf4a7fb99722d084dce6e1d22c9229db4ee01