Don't send volume keys twice

Introduced by CEC VOL_ADJUST_* changes which send a volume change event
for key up with the intent that this will be passed on to CEC. We want
to ignore it if it's a KEY_UP event and we're not using CEC.

Change-Id: Ib001b1f00bd1117f5e60aeb28df8461a00436678
Test: Manual: just press the MUTE button and release it (and the device mutes)
Bug: 158119517