Add scrim under status bar and above thumbnails.

For Recents Go, the thumbnails actually go directly under the status bar
text, so we should ensure that there is some scrim to ensure it's always
readable. We do this by adding a foreground drawable scrim to the
activity's root view.

Since this root view is used in other Launcher states, Launcher has to
toggle this on and off when going/leaving recents. When this view is
just being used for recents as a stand-alone, however, (i.e. when the
user has a different default launcher), it's fine to set this once at
the beginning since nothing else uses the root view.

Bug: 131834685
Fix: 131834685
Test: When quickstep is the default, go to recents and back. Status bar
is readable. Scrim does not animate when going to and from recents.
Test: When quickstep is not the default, scrim shows up fine
Change-Id: I5d88a1fd873860887b3141e1ec0ee999f59ccfad