Clean up LetterboxTaskListerner and all related logic.

This is the first step towards moving letterbox gravity logic from WM Shell to WM Core. It's important to do this even before the new implementation in WM Core is ready because surface repositioning in WM Shell causes visible flickering on rotation for all apps on devices with cutout.

Fix: 174632285
Bug: 175212232
Fix: 173440321
Test: atest WMShellUnitTests
Change-Id: I2b3706a1a46bb2c7ee4bb3789fb09b216e32f290