Fix back gesture / keycode for the PlatLogoActivity

By overriding onKeyUp on the Activity itself, without
calling super, I broke legacy back handling. Fortunately,
there is now a better way to handle back!

(Also fix the theme to avoid wallpaper showing through
display cutout insets.)

Test: adb shell input keyevent --longpress KEYCODE_BACK
Test: swipe from edge
Fixes: 289648693
Fixes: 290989795
Change-Id: I4fd4975421d204f06dbd8acda1a0d2a5a99f3465
(cherry picked from commit 5bbab52e)