Show instant app notifier for non-fullscreen activity

On the original Android, an instant app notifier shows only for
fullscreen and split-screen-secondary windowing modes.

This CL adds freeform windowing mode for ARC.

(manually cherry picked from commit
18aa4446bc049ae6592828f5f646ad421958208f in vendor/google_arc)

Bug: 127858333
Bug: 128961725
Bug: 123917211
Bug: 128877314
Test: manual (run an instant app and confirm a notification)
Change-Id: I3fdc8e5ccfae479082dc3fa287e0b77cbc1ecc4a