[fenix] Bug
1812518 - Show the download dialog as an Android View
Tried to mimic the UX of a modal dialog while using Android Views.
This meant including a scrim that would consume all touches and theming the
navigation bar and status bar.
Avoiding a dialog and a separate window will allow the snackbar to see the
new "dialog" as a sibling in a CoordinatorLayout parent and so be able to
position itself based on the new "dialog".
This patch also added "start_download_dialog_layout" from A-C as it leads to
simpler and less code needed to style the layout - colors / shapes with
everything happening in XML versus calculating the values then setting them
programatically.