Clear individual notification by swiping left or right.

The user now has the ability to clear individual notifications by swiping to
the left or to the right. Only notifications which are marked as clearable
can be cleared (ongoing notifications, for example, can not be cleared).

Bug: http://b.android.com/10831
Change-Id: Ic62967456e83d46ebc474031b0a8a84f5f7f9716