Use top GrantPermissionActivity to open activities

If there is an invisible GrantPermissionsActivity on top of another, use
this invisible, top activity to start other activities, because only the
top activity can receive results. Also makes delegated activities share
the viewModel of the main activity.

Fixes: 267754399
Test: manual
Change-Id: Id1bd46d852808c9c683321fcd96266a4a6ac30be