Letterbox: make letterbox work with drag resizing

Fixes an issue where letterboxing disabled using the insetFrame for
drag resizing, leading to unsightly jank when doing so.

Also makes sure that the insetFrame is always cleared when
not in multi-window mode, because that could lead to stale
frames when leaving a non-fullscreen task left multiwindow
mode.

Change-Id: I5576fe86ce17f8cd377bc6a55b3d50d7703569a0
Fixes: 76009001
Bug: 64369819
Test: Enable double cutout, enter landscape, enter splitscreen, drag handle, verify no weird layout issues.
Test: Open ApiDemos > App > Activity > Max Aspect Ratio > 1:1, enter split screen, open other app, drag handle up a bit, drag handle all the way up. Open Api Demos activity again from recents, verify it has correct size.