Allow display letterbox if starting or base application

If there are two windows of TYPE_APPLICATION_STARTING and
TYPE_BASE_APPLICATION, we should allow letterbox to be shown if one of
the windows are letterboxed even though it's not on the top z order.
This will prevent flickering when the letterboxed window is brought to
top.

Bug: 317319213
Test: atest SizeCompatTests
      Open YT video in explore page
Change-Id: I4fcf4785a6b6a50e7baeabf28efd83a3dd11b7f4