[flexiglass] runPostCollapseActions from the main thread

Avoids a fatal exception that gets thrown downstream when running
the action from a background thread.

Bug: 308819693
Test: atest ShadeControllerSceneImplTest
Test: tap on a notification from the shade over the lockscreen;
observe no crash
Flag: com.android.systemui.scene_container

Change-Id: If79e3485c42f3ae4ecfa2bcd11e181941a7b862a