Avoid repeated/redundant work.

1. Avoid scheduling the idle alarm if the device is already considered
   idle.
2. Avoid scheduling the idle alarm again if the currently scheduled
   alarm is for the same time.

Bug: 309292340
Test: atest FrameworksMockingServicesTests:DeviceIdlenessTrackerTest
Change-Id: I0cb49041d15e5b2bd880ddc06cb2f1aa95e8c174