Add test for device type helper functons.

The added tests cover:
1. audio_is_remoute_submix_device
2. audio_is_digital_in/out_device
3. audio_idevice_is_digital

Test: atest systemaudio_tests
Bug: 135621476
Change-Id: Idf8a9db97db6035ecc832a6a9bba6f64f8fc7479