[2/n] TransparentPolicyState definition

Incapsulate the state the transparent activity inherits from the
opaque one, into a TransparentPolicyState object.

The TransparentPolicy for a given ActivityRecord is running if
it has a valid TransparentPolicyState.

Flag: EXEMPT refactor
Bug: 337346942
Test: atest WmTests:SizeCompatTests
Test: atest WmTests:LetterboxUiControllerTest

Change-Id: I60d9387f4f50246dc61a4168d45f559a43e66877