TF700T: Support volume mute keycode

Use asusdec for capture the volume mute key. KEYCODE_VOLUME_MUTE is part of the
aosp keyevent intercept handling, but aosp uses it to stop ringing in phone
devices (no as system volume mute toggle). Since transformer devices doesn't
have a telephony subsystem, this class handle and treat this event as a volume mute
toggle action. Since this key event could be also consumed by an application,
the asusdec KeyHandler mustn't mark the key event as consumed.

Change-Id: I82cc74454ff44c04004fa18e9027651e783d0c6e
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>