Make VolumeDialog respect the mouse cursor.

Mouse hover is usually used to indicate user's attension on
transient UX like the volume dialog. As is discussed on the bug,
VolumeDialog should extend (reschedule) the dismiss timeout
upon the hover events, so that it keeps visible as long as the
user pays attension to the dialog.

Bug: 22558848
Change-Id: I7e1166b7673bbdc97bfdc5472dea994715a653fd