Sorry for replying late, but I wanted to add more to this thread.
A thread about Nvidia Shield and Reicast Emulator was posted here
Widescreen works for me, but I had to download the latest compiled beta build here
's how to install .apk files.
If games do not load:
1. Uninstall reicast
2. Download the apk behind the one you downloaded
3. Install it
Updates may break the app, so you have to do this.
Once you get reicast installed
1. Press the 3 Lines on the right
2. Go to the Path Tab
3. Set the paths of your roms and bios.
4. Go to Input Tab
5. Press Select by controller a and press a button on the nvidia shield controller
6. Go to Settings tab and Enable Widescreen mode
7. Go to Browser tab and try a game.
Widescreen mode was not available in the original dreamcast, so it is loading extra data not intended to show, so it may not look correct.
That's all. Reply if you have any trouble