Add flag for always force-consuming the captionBar

To avoid overlap issues with apps that don't handle captionBar insets
well, Desktop Windowing may set a flag on its caption inset source that
will make DecorView consume the inset for the app.

Bug: 352563889
Test: m
Flag: EXEMPT adding a flag
Change-Id: I86654049c82b16c4ee21668498854c7089668200