[61/n] Always use Single Letterbox Surface for Bubble

Bubble have rounded corners and it's easier to use a single
letterbox surface.

Flag: EXEMPT Refactoring
Bug: 423072242
Test: atest WMShellUnitTests:LetterboxControllerStrategyTest
Test: atest WMShellUnitTests:TaskInfoLetterboxLifecycleEventFactoryTest
Test: atest WMShellUnitTests:LetterboxLifecycleControllerImplTest

Change-Id: I0c28b4336747f453bebfb44a91e58a2823dfbbd4