Fix incorrect flagging of flow change.

I used apply instead of let, which meant that, since flowOn and
stateIn/shareIn don't actually change the object, but instead wrap it
and return something else, the apply wasn't doing anything at all :(

Flag: com.android.settingslib.flags.volume_panel_broadcast_fix
Test: v2/android-crystalball-eng/health/microbench/systemui/main/systemui-statusbar-2-jank-suite
Fix: 348810424
Bug: 347707024
Change-Id: I2f8ddf45ebe32a4c8deca9a079734daef66d4b21