Fix hit testing inside layers.

Fix hit testing inside layers.

* Prevent asking for a sync in GraphicsLayerAndroid if some property has not
  changed.
* Remove scrolling logic from LayerAndroid and create a subclass for scrollable
  layers.
* Report the scrolling limits to the layer in order to scroll iframes (not
  turned on) and to avoid computing them each time the layer is scrolled.
* Change the foreground rect calculations to better match the non-overflow case.
* During hit testing, intersect the hitTestClip with the foreground and
  background to prevent false positives in the layer.
* Prepare for iframe scrolling by adding code to trigger compositing for
  iframes.  This currently works great except for navigation so it disabled for
  now.

Bug: 3258631
Change-Id: I0da2d8dbe25376c6aa4f485c9350048c82c6f563