Flexiglass: Fix QSPanelControllerTest when flexiglass is enabled

onInit_setsMediaAsExpanded is failing when flexiglass is enabled because
mediaCarouselInteractor is now used in the init step. Thus we need to
set it up correctly.

Also, certain code now needs to run on the main thread so that
JavaAdapterDisposableHandle can run properly.

Bug: 416716751
Test: QSPanelControllerTest
Flag: TEST_ONLY
Change-Id: Idc0da13aabb57cbd2ece1c34fe1b3b9bb402d637