Expose Usage Setting API

Expose APIs as follows:
-A CarrierConfig key that allows carriers to override the
 device's usage setting.
-An API that allows callers to query the current usage setting
 for a given subscription.
-A set of constants to describe the usage setting.

Bug: 210023167
Test: atest SubscriptionManagerTest#testCellularUsageSetting;
Test: atest CarrierConfigManager#testGetConfig
Change-Id: I62534d6f304a297f646d1dd4b008a9a4d4a30560