Recents: Clear all button for recent apps.

An icon will appear on navbar when there are apps available to be killed on recent apps.

Credits to @BigBrother1984 for kitkat porting from jellybean

Patchset 2: Tablet mode is deprecated now, so no need of multiple callbacks
Patchset 3: Remove copyright from unaffected files
Patchset 4: Remove unused imports

Change-Id: I93d1d4cb34325190b03ae3a0d40e1bc170c92d91