Return DeviceDescriptor or DeviceVector in Engine.

To remove the limit on the number of audio device types, there is a
need to remove the return value as a combination of audio_devices_t.
Using DeviceDescriptor or DeviceVector in Engine is a starting point.

Test: audiopolicy_tests, smoke test
Test: CTS for AudioTrack, AudioRecord, AudioManager
Bug: 135621476
Change-Id: I7603481d92184549a5fed6b1166cb0044c965570