[11/n] Implement LetterboxController for multiple surfaces

Create MultiSurfaceLetterboxController to use when required by
a specific strategy (e.g. when rounded corners are present or
when the activity is transparent). The strategy will be defined
in a follow-up CL.

Flag: com.android.window.flags.app_compat_refactoring
Fix: 377857898
Test: atest WmTests:AppCompatUtilsTest

Change-Id: Iae4d0e852b4ee645e1730886da31b2fef54bd1e9