Open duration dialog from the main thread

Dialogs expect to be created from the main thread, so let's make sure we
switch to main when opening the duration dialog from the quick toggle on
the modes quick settings tile.

Fix: 391385776
Flag: android.app.modes_ui
Flag: com.android.systemui.qs_ui_refactor_compose_fragment
Test: turn on "Ask every time" and check that dialog works correctly
Change-Id: Ia0b086fd3e92f31a1ba9d5a75ae7a4d9ff7f6734