hide action bar from launchActivity
Adding sushi background to launchactivity.

Some details: I've added ldpi/mdpi/hdpi/xhdpi assets, cropped for
appropriate resolutions. I initially created .png assets, but the source
is from a 3MB jpg stock photo (maybe uncompressed, but still a lossy format so I don't trust it). Using .png put me at 2MB for the image assets, which is
unacceptably high. I think one of the main advantages of .png is alpha
transparency, and I don't need transparency as it's a background image.
With a high quality JPG (only 80% compression) we're still sitting at near
500kb, which is a bit high, but I'll play around with things more once I'm
comfortable that this won't look ugly on any tested displays.