Allow DeviceStateProvider to provide optional String name for device states.

This CL updates the DeviceStateProvider interface to allow a provider to
supply device states that contain both an integer identifier and an
optional string name. The name is used alongside the integer identifier
when the state is printed in a human-readable form, for ex over ADB.

Test: atest DeviceStateProviderImplTest
Test: atest DeviceStateManagerServiceTest
Bug: 159401801

Change-Id: Ie23f5df4609dd9e5c67eb283f026cee57a788ee6