Adapt FinishActivity and outro for 14 QPR3

windowShowWallpaper behaves unexpectedly with transitions as of 14 QPR3.
As a result, the changes of "Revamp outro animation" were causing the
transition into the final page of the wizard to instantaneously have
a transparent wallpaper background, even before the animation finished.
This was jarring and odd, so now we use a regular theme until we are
ready to start our animation, at which point we recreate the activity
with the EdgeToEdgeWallpaperBackground (windowShowWallpaper) theme
before running the animation.

Change-Id: Ib19918d7d2a615aff44a48066b17ec9d2f04bac6