[1/n] Create TransparentPolicy and move related code

As first step we create the TransparentPolicy class and move the
related code from LetterboxUiController to it.

Flag: EXEMPT refactor
Bug: 337346942
Test: atest WmTests:LetterboxUiController
Test: atest WmTests:ActivityRecordTests
Test: atest WmTests:SizeCompatTests

Change-Id: I5452101cd5fa8f82b6faf4455d4eceeff4d48f8c