Fix up 'clear recents' UX.

Use the 'dismiss all' button drawable and place the button in the upper
right corner. Doing so fixes the broken animation when going into
recents.

Change-Id: I497a8b023f1872c5f9ae5df87af6ed2159e6b7b1

Conflicts:
	packages/SystemUI/src/com/android/systemui/recents/views/RecentsView.java