[tp] Changes GlobalScope to CoroutineScope(Dispatchers.Main) (2/2).

See bug for more explanation.

Bug: 279761972
Test: manually verified that the wallpaper picker UI comes up, can be
navigated beteween home and lock screen tabs and that entering every
single section and existing does not produce any crashes. Also changes
every option and reverted through the undo system.

Change-Id: Iac3d1f581b4b13ef2044d3c985f87b61c8c43975