Letterbox positioning (3/3): Configurable horizontal position.

This CL finishes letterbox positioning changes started in ag/13575049 and ag/13780285.

Fix: 1752121232
Test: SizeCompatTests and manual with an ADB command
adb shell cmd window set-letterbox-horizontal-position-multiplier <multiplier>

Change-Id: I0690bd6de0d476487eaab48ad6f0583bdeaf0c2b