Refactor SplitBounds to always use creation-time measurements

In flexible split ratios like 90:10 and 10:90, we hide the Overview icon of the smaller app completely (ag/30949010). However, there was an unintentional side effect where one of the icons would disappear when rotating the display.

This happened (basically) because SplitBounds is not recalculated on device rotation, so fields like leftTaskPercent and topTaskPercent should not be used directly when the rotation state is not known. I added an API to SplitBounds that hopefully makes it harder to write bugs like this in the future.

Fixes: 395783367
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: Icons do not disappear on rotation. When app chip menus are enabled, the (existing) correct behavior is not changed.
Change-Id: I2c1b38f4bd9a76dfb127b7bbf230897d747d3c49