Add delayed Standby for Playback devices on hotplug out

Upon receiving a hotplug out event, a Playback device starts a 30 second
countdown. If no user interaction with the device occurs during that
time, it goes to Standby. Otherwise, it will check again every
30 seconds until a hotplug in event is received.

Test: atest HdmiCecLocalDevicePlaybackTest; manual
Bug: 206934433
Change-Id: If4a9f62c7ace93ee82a28c875d2efc7fa1dd635f