Hide overlay windows when showing phone account enable/disable screen.

Hide any system alert window overlays when the screen that lets the user
enable/disable phone accounts is shown.

Test: Manual test with overlay shown from test app; verify that the overlay
is hidden when the phone account selection screen is opened.
Bug: 246933359

Change-Id: Ia0209d57ee9a672cde4196076845d77941dc3f68
(cherry picked from commit a7d57ace)
Merged-In: Ia0209d57ee9a672cde4196076845d77941dc3f68