Fix RNCamera bug

[zbreit]

Currently, the react-native-camera module has an issue where the preview will render correctly the first time an emulator boots up, but then it will just show a black screen on all subsequent renders. I don't believe this is an issue on physical devices but can't test that theory.

The likely cause of this error is that react-native-camera doesn't deal well with multiple cameras or emulators. It is recommended that you unmount the <RNCamera/> component every time its screen loses focus, which we actually do. You can see the native errors being caused by running

$ adb logcat | grep camera

Attempt to fix this issue on emulators, or ensure that it is not a problem on any physical devices.