Add clear all recents action to recents panel

Change-Id: I2243ebc83abc483e78b112900a653d7e61edc5ba

Conflicts:
	packages/SystemUI/res/values/strings.xml
Fix - place Clear Recents button above the nav bar

Change-Id: Ie7059cd1f71bae28ff25f367221b7c7ba0131932
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