Fix letterbox insets when starting app

Showing the letterbox with checking on HAS_DRAWN is too late. We
also need to show it in READY_TO_SHOW such that we have it
available when starting the transition.

Test: Open letterboxed app. Observe no flicker
Test: Capture winscope trace and make sure content insets when
reopening app are correct.
Bug: 76220728
Change-Id: I20ed8b1b5a90cc0a878d3eb1512e1aa1e4cd37f3