Scale screenWidth for split proportionally for thumbnail matrix

* Instead of passing in screenWidth to be that
of entire screen when using split, scale screenWidth
to that of only the amount that the given split app
is taking.
* Created TODO(b/254378592) to consolidate different
SplitBounds classes

Fixes: 250817893
Test: Checked phone/tablet swiping quickswitching
portrait and landscape

Change-Id: I5c3b2076110e358366284b2c794fd8df8a79f034