[19/n] Make LetterboxControllerRobotTest more generic

Now the LetterboxControllerRobotTest is completely decoupled from
the way the specific LetterboxController implementation is created.

Flag: com.android.window.flags.app_compat_refactoring
Bug: 382423480
Test: atest WMShellUnitTests:SingleSurfaceLetterboxControllerTest
Test: atest WMShellUnitTests:MultiSurfaceLetterboxControllerTest
Test: atest WMShellUnitTests:MixedLetterboxControllerTest
Test: atest WMShellUnitTests:LetterboxUtilsTest

Change-Id: I5ff2174fd275e0f89ab5602167c7cf37386eacf5