Fix volume key events are dispatched when screen is off

This fixes an error introduced by CL1736940 that the ringtone stream
volume is being adjusted when user presses the volume key during screen
off.

Bug: 36430174
Test: volume key up/down when screen is on and off
Change-Id: I78e957e2f87940a22a99b80f4ed65d986bc130c7
Signed-off-by: David Lin <dtwlin@google.com>