Enable tests in HdmiCecLocalDevicePlaybackTest

The tests were marked as `@Ignored`, however are valid and passing.
This change re-enables three additional tests around System Audio Mode
handling for Playback devices.

Test: atest HdmiCecLocalDevicePlaybackTest
Bug: 120845532
Change-Id: I4e890fa922cdbb1c552c0384b5548de7a9b9f987