Use black theme without actionbar for RoutingActivity

The theme from the manifest is used for the startup animation.
Previously light background with a dark actionbar and the
green icon was displayed during the startup animation, then
the theme switched to whatever is configured in settings.
Now a black background without actionbar is used for the
startup animation. This was chosen because a black "flash" when
using the light theme is visually less distracting than a white
flash when using the dark theme.