[70/n] Improved Strategy for Translucent activities

Always use a single letterbox surface when rounded corners are
enabled unless the activity is translucent.

For translucent activities we always use multiple surfaces
without rounded corners

Flag: EXEMPT Refactoring
Bug: 377875146
Test: atest WMShellUnitTests:LetterboxControllerStrategyTest

Change-Id: Ib779b15eb99eae0e46dccc50b7fab50ca0f8fca2