Exempt from app standby logic

When AppOpsManager.OP_SYSTEM_EXEMPT_FROM_APP_STANDBY is set to
AppOpsManager.MODE_ALLOWED, app should be placed into
UsageStatsManager.STANDBY_BUCKET_EXEMPT. Otherwise app standby buckets
should behave as normal.

Test: atest AppStandbyControllerTests
Test: Manual testing:
	Install test dpc and set as role holder.
	Navigate to "Set Application Exemptions" in Application
	Exemptions category.
	Choose app and select "Exempt from App Standby" and click save.
	run "adb shell am get-standby-bucket packageName" where the
	package name is the name the app selected previously (can be
	viewed in the toast followed after setting the exemptions).
	Returned value should be 5 (standby bucket exempt).
	Navigate back to "Set Application Exemptions" in testDpc and
	save exemptions with "Exempt from App Standby" unchecked.
	Running "adb shell am get-standby-bucket packageName" should now
	return a value other then 5.
Bug: 246330879
Change-Id: I8f28a15552e524cbb9cbd2c8c23fd6026d9341f9