[34/n] Migrating tests for AppCompatLetterboxPolicy/Overrides

Migrating existing tests to the Robot pattern.
Deleting LetterboxUiControllerTest

Flag: EXEMPT refactor
Bug: 336807329
Test: atest WmTests:AppCompatLetterboxPolicyTest
Test: atest WmTests:AppCompatLetterboxOverridesTest

Change-Id: Ie116007ba038fc411e82b16f541be03ecf5466de
7 files changed