Remove and restore selection for non-isolated dialogs

In non-isolated mode the only way to prevent accidental typing in
the document is by removing the native selection, so do this.

This unavoidably sets the model selection to null, so when the dialog
closes, restore the selection if it hasn't been already by an action.

Bug: 73097
Change-Id: I93f741734ff3d46397c1278c500ae3c124a3a14f