Rename layoutLetterbox() following existing conventions

The layoutLetterbox() method can be a no-op on some conditions
so the right name should be layoutLetterboxIfNeeded()

Same for updateLetterboxSurface() renamed as
updateLetterboxSurfaceIfNeeded()

Bug: 324417794
Test: atest WmTests:LetterboxUiControllerTest

Change-Id: I1ad7f11e4b16a9b2b8b0e2d46c5ddcf1ead6de84