Prevent play/pause key from starting voice search on TV

CDD enforces the TV devices' remote controllers to have search button,
so long-press of play/pause key to start voice search doesn't make
sense.

Test: Manual, and run CTS (MediaSessionTest)
Bug: 63139504
Change-Id: I757ad4a9f1d9677cefc6403a96c5bf6c7323b575