Prepare screen overlay warning for targeting API 30

This MR removes the ability to show and remember a list of apps drawing on screen as we can't query all installed apps anymore when targeting API 30. The warning dialog on API 30+ has been modified slightly to accommodate for this. An action was added to review the list of apps allowed to draw on screen.

When a permission is revoked and the user returns to Briar, the screen overlay is gone immediately, allowing the user to interact with Briar again.

device-2020-12-14-105516

First part of #1753 (closed)