Fix reporting of unchanged keep clear areas

ViewRootRectTracker#computeChangedRects returns all tracked Rects if
there are changes to the Rects since the last call to that method, or
null if there aren't.
When ViewRootImpl checked for changed Rects, if only one of either set
of restricted or unrestricted keep clear rects changed,
then #computeChangedRects returned null for the other, which was
subsequently reported as an empty list. If there were keep clear rects
previously reported, this would mistakenly clear them.

This change separates the check for changes and retrieving the list of
latest Rects. If either set of Rects has changed, now it's always the
latest Rects that get reported.

Bug: 231532058
Test: Manual with app with restricted & unrestricted keep clear areas
Test: atest KeepClearRectsTests
Change-Id: Ida51420d0f0e7935a5dc9f3eab902857927c33f5