Hideybar gesture clears flags if SYSTEM_UI_FLAG_FULLSCREEN

If an app uses the system-clearable SYSTEM_UI_FLAG_FULLSCREEN to hide
the status bar, swiping from the top of the screen should clear the
clearable flags, revealing the system ui, and allowing apps to respond
to the change.

This differs from the behavior the gesture triggers when an app hides
the status bar using the non-clearable WM.LP.FLAG_FULLSCREEN.
In this case, we temporarily overlay the transparent auto-hiding bar.

Bug: 8682504
Change-Id: Ie5dc885fd13c8d980c136f6babcea181565ef2a2