Fix crash when trying to open a Mode's settings from lockscreen

The general issue is a Compose bug where dismissing a Dialog during longClick crashes (silently, to boot!) and then is left in a nonresponsive state (b/394571336). So as a workaround we post on the main thread instead of doing it immediately.

Fixes: 394241061
Test: atest ModesDialogDelegateTest + manual
Flag: EXEMPT Simple bugfix
Change-Id: Ie028e135498b2159e7e92145921ae6a82b26e431