Add flag for caption inset conversion for app compat

To guard app compat treatment targetted at apps that don't handle
captionBar() insets correctly, but do support other inset types.
This is aimed at preventing overlap between the caption bar and the top
of the app content in freeform mode.

Bug: 316231589
Test: m
Flag: EXEMPT adding a flag
Change-Id: Ib438b428aab22ce201ea51918bf4bdf3c090f530