[29/n] Encapsulate Letterbox state in Policy

Encapsulate letterbox logic in a policy local state
managing the Letterbox object which is the one owning
the actual letterbox Surfaces.

Flag: EXEMPT refactor
Bug: 356385137
Test: atest WmTests:LetterboxUiControllerTest
Test: atest WmTests:SizeCompatTests
Test: atest WmTests:AppCompatUtilsTest

Change-Id: Ide2d8df5f4b4c33f80e6c729ce73d15a4e57ec7d