Expose App Standby APIs for apps and system

Allow apps to query their own standby bucket.
Allow privileged apps with PACKAGE_USAGE_STATS permission to
query apps.
Allow privileged apps with CHANGE_APP_IDLE_STATE to set the
standby state for apps, but not for themselves.
Removed AppStandby class and moved constants into UsageStatsManager.

Bug: 63527785
Test: cts-tradefed run cts-dev -m CtsAppUsageHostTestCases
Change-Id: I3c1c20f6ecb6d54e248233696039286b243d663c