Set content of ComposeView after setting back owner

The way the code was written before, there was no guarantee that the
content of the ComposeView would be added and composed before we set the
back dispatcher, and that is needed for the STL.

This way, the content should always be set after the back dispatcher is
set.

Test: manual, no changes as we couldn't repro crash
Fixes: 370869111
Flag: com.android.systemui.qs_ui_refactor_compose_fragment
Change-Id: Ie58955b96a973fcc2cced91e9781a178f4a2efe5