Add shell am command to set and get app's background restriction level.

Added two "adb shell am " command.
adb shell am set-bg-restriction-level [--user <USER_ID>] <PACKAGE> unrestricted|exempted|adaptive_bucket|restricted_bucket|background_restricted|hibernation
adb shell am get-bg-restriction-level [--user <USER_ID>] <PACKAGE>

Note: right now only when the restriction level toggle between above(include) adaptive_bucket level and below(include) restricted_bucket level, the actual app standby bucket would change.

Bug: 211694922
Test: adb shell am get-bg-restriction-level --user 0 <packageName>
adb shell am set-bg-restriction-level --user 0 <packageName> restricted_bucket
adb shell am get-bg-restriction-level --user 0 <packageName>
adb shell am set-bg-restriction-level --user 0 <packageName> adaptive_bucket
adb shell am get-bg-restriction-level --user 0 <packageName>

Change-Id: If9cc2b0b0e3acaf2f145f79c9bf5072054994114