Overlap navbar with letterboxed immersive app

When immersive app is letterboxed, it has a large space not occupied
left at the bottom of the screen in 3 button navigation mode. This is
due to transient nav bar insets not included to calculate
positionMultiplier. To overlap app with navbar, bounds should be used
instead of appBounds.

Fix: 277543176
Test: Manual && atest WmTests:SizeCompatTests
Change-Id: I93f40e4d23193485015c26c3ec3f808675d1cd64