Make onClickHandler in KeyguardHostView a weak reference

The strong reference OnClickHandler caused bitmaps in keyguard to be
referenced when keyguard was not visible. This change makes the click
handler a static class with a weak reference to the hostView instance,
allowing those bitmaps to get collected and shaving ~845k off of the
heap size when the keyguard isn't showing.

Issue #10918599 SystemUI should have a round of Svelting

Change-Id: I69de8659ac14c1a4723d082dd3cd394d8b6097f1