Don't shown letterbox surfaces unless activity is visible.

Also, merging visibility and "surface ready" checks in LetterboxUiController#shouldShowLetterboxUi because other UI parts of letterbox like rounded corners and background also shouldn't be applied until those conditions are true.

Fix: 185955096
Test: atest WmTests:SizeCompatTests
Change-Id: I5822d651ff65c17fce4891b8728546bec235e54f