CEC: Revive a standby handler

When a device is going into standby state, every CEC message is supposed
to be handled by HdmiCecStandbyModeHandler. But the messages were
blocked by address allocation check and just buffered. This change
revives the standby handler so that the device can handle messages
properly during standby mode.

Bug: 30184027, Bug: 31873357, Bug: 33095971
Test: One touch play with chromecast works well during standby mode
Change-Id: I3315fdaee346838c6a2c68bad0ac64b94ab9723c