Add sendVolumeKeyEvent API to handle forwarding volume key separately.

Note that we handle volume key the same way as other keys before when
forwarding them to other cec devices. But the destination device of
volume key might be different from other function keys.

We might need to take the System Audio Control logic into this
forwarding proccess to find the proper audio receiver address.

Test: make -j44
Bug: 123369653
Change-Id: I6c9dba2b333e7eaa5137a8d2f5bfed506ae8554b
(cherry picked from commit 5db0138297836baf570f354c0b2ecb988d30936b)