[16/n] Use rounded corners in LetterboxControllerStrategy

Used rounded corners radius in LetterboxControllerStrategy
as initial criteria about using a single or multiple surfaces
for letterboxing in Shell.

Flag: com.android.window.flags.app_compat_refactoring
Bug: 377875146
Test: atest WMShellUnitTests:LetterboxControllerStrategyTest

Change-Id: Id2c2ad8c9e344bb58710254125459b0f490554bd