Clear All button added in  Recent Panelview.

Ref: https://github.com/temasek/android_frameworks_base/commit/a129580ee886b48c124de716236c2bae2b93a77b

PS3: capability to show/hide based on setting added

Change-Id: Iab4db7abec6f8caee4c0faad0960fa0f9fb9a1fd
PS6: capability to change button layout_gravity

Conflicts:
	core/java/android/provider/Settings.java
	packages/SystemUI/src/com/android/systemui/recent/RecentsHorizontalScrollView.java