[12/n] Implement tests for LetterboxControllers implementations

Implements Robot to test LetterboxController implementations and
implements tests for Single and Multi surfaces.

Flag: com.android.window.flags.app_compat_refactoring
Fix: 377857898
Fix: 370997904
Test: atest WMShellUnitTests:SingleSurfaceLetterboxControllerTest
Test: atest WMShellUnitTests:MultiSurfaceLetterboxControllerTest

Change-Id: I91b55903fbe4e5be962152cff20b948393d8be26