Don't join multiple app-triggered permission dialogs

If we do merge multiple app-triggered dialogs, there's a chance that we
can end up merging a dialog into a dialog that's now hidden behind an
activity. The merging code is really for system-triggered dialogs, so
restrict it to that.

Fixes: 250101648
Test: manual
Change-Id: Ibb9e18d59f53663683f898675b9dd47fa18ab495
(cherry picked from commit eaf3a339)