Add a method to flush AppStandbyController's handler

And flush the handler in testing to guarantee correct results.
Also, fix AppStandbyController#setIdle by removing any existing bucket
expiry times that could immediately change the idle status.

Test: atest AppStandbyControllerTests
Bug: 185169504
Change-Id: I7ec22e4eeffca18927531f62edffcc51243fb648