App Bucketing for Standby

Manage the standby bucket in AppStandbyController

Default implementation of bucketing based on simple timeout:
 ACTIVE, if recently used
 12 hrs to move to WORKING_SET
 2 days to move to FREQUENT
 7 days to move to RARE
 (subject to change)

RARE bucket equates to the old "idle" or "inactive" state for
an app.

Bug: 63527785
Test: AppStandbyControllerTests.java
Change-Id: I970d7afcdf47c31a9413da8fd4852066a13676a2