Launch voice assistant by the KEYCODE_MEDIA_PLAY_PAUSE

Android's headset spec is update to deprecate the KEYCODE_HEADSETHOOK,
and the KEYCODE_MEDIA_PLAY_PAUSE will be used instead. So we need to
launch voice assistant by both the KEYCODE_MEDIA_PLAY_PAUSE and
the KEYCODE_HEADSETHOOK.

See following docucument for the details
https://source.android.com/devices/accessories/headset/usb-headset-spec#software-mapping

Bug: 36575213
Test: Manual test (injects long press of the KEYCODE_MEDIA_PLAY_PAUSE
    and confirmed that it launches voice activity)
Change-Id: I3dc2500ca5bb3d9d0ba3104246a19b84e731b46f