CEC: Check standby action availability before request for sleep mode

When the playback device is asked to go to Standby state with <Standby>
message, it needs to check whether it can accept the request or
not. This check should be done before asking the system for sleep mode.

Additionally, this change will fix the behavior after screen
timeout. After the timeout, playback device may go into sleep mode, but
canGoToStandby() check will prevent the device from tracking the right
current power status value. So need to set the status properly, and keep
HdmiCecLocalDevices enabled for accepting future CEC commands.

Bug: 30182748
Test: Check fugu screen after turning off/on Samsung TV and
Archer. Also tested the case of screen timeout.
Change-Id: Idbcd7abd5d4e22942845760f77f7f7e622b987c0