Retry AppStandbyControllerTest bucket checking

AppStandbyController offloads work to a seperate thread which can lead
to race condition induced flaky test failures. Retry bucket checking for
up to ~two seconds to reduce the flake chance.

Test: atest com.android.server.usage.AppStandbyControllerTests#testUnexemptedSyncScheduled --rerun-until-failure 10
Fixes: 163385558
Change-Id: Iedb9ccb1d834291ac62748e622ba1895f5a9ef43