Resume RT-animations after a pauseSurface

Bug: 18203577

The issue occurs as a result of performTraversals() both doing
a window relayout call *and* early-returning because it's not dirty.

To fix this pauseSurface() returns whether or not the RT-side is
"dirty" to force ViewRootImpl to do a draw even if mDirty is
otherwise empty.

Change-Id: I534f367e75d18d273ebf14df3927f5c464ef6bef