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

src/com/android/launcher3/util/SplitConfigurationOptions.java

@@ -100,6 +100,7 @@ public final class SplitConfigurationOptions {
      * with the same name/functionality in wm.shell.util (which launcher3 cannot be built against)
      *
      * If you make changes here, consider making the same changes there
+     * TODO(b/254378592): We really need to consolidate this
      */
     public static class SplitBounds {
         public final Rect leftTopBounds;