Add the API surface for DeviceConfig.

Update DeviceConfigService to call DeviceConfig API directly for get,
set, and reset. Remove the duplicated content uris from various places
in favor of the single constant exposed in DeviceConfig.

Test: atest FrameworksCoreTests:DeviceConfigTest
      atest FrameworksCoreTests:SettingsProviderTest
      atest SettingsProviderTest:DeviceConfigServiceTest
Bug:109919982
Bug:113100523
Bug:113101834

Change-Id: I46d110c4fd29a89af383629d26de4ee39ca852a6