Maintain display power status for playback device

With this CL, the power status of TV/display is keep tracked of
by hdmi cec server part, specfically HdmiCecDevicePlayback.
Turned the HdmiCecDevice to abstract class from which classes of
concrete device type(HdmiCecDevicePlayback, HdmiCecDeviceTV) are
inherited. The display power status code is put in HdmiCecDevicePlayback
only. HdmiCecDeviceTv will have its own logic that manages power status of
devices connected to it. For now it only has a bare minimum code.
Will be worked on in follow up CLs.

Other changes:

    - Replaced sendGiveDevicePowerStatus() with isTvOn() so that the status
      can be queried by clients.

    - Defensively check the availability of HdmiCecService so that
      HdmiCecManager.getClient() returns null in case the service couldn't
      be initialized. This ensures clients of the service gets the nulled-out
      HdmiCecClient when called in the state/configuration where the service
      is not available, thus proceed accordingly.

Change-Id: Idaf69e73cfbd639c0b40b1bd4b6146f011246180