Force non-floating main app windows to not fit anything

Since we have a better approach of fitting content instead of
setting them on the layout params
(Window.setContentOnApplyWindowInsetsListener), we can now
simplify forcing of filling the screen:

Instead of clearing the fit types when the private flag is set, we
unset the types and straight out reject any params when the client
would like to fit.

Test: DisplayPolicyTests
Bug: 118118435
Change-Id: I845e6b1c81e29ab66a770891d03c62a32418e8cc