Adds DeviceConfig.getProperties method for atomic reading.

This fetches multiple flags atomically, either the entire contents of
the namespace, or just a list of flags specified by the caller. This
update also enables the local Settings class to fetch and cache the entire namespace
whenever any flags from that namespace are read so that we only incur the cost of the
IPC once.

Test: atest FrameworksCoreTests:DeviceConfigTest
      atest FrameworksCoreTests:SettingsProviderTest
      atest SettingsProviderTest:DeviceConfigServiceTest
Bug: 136135417
Change-Id: I0be7c4b51c37590f5001e53b074b06246851a198