Make sure focus points are valid

A `RejectedExecutionException` is thrown when you attempt to set the parameters of the camera after setting a new focus area, then zooming. I'm not sure how you'd like to handle this. I've only tested this on an HTC One.

To reproduce:
1: Open the app and gain a new focus area
2: Pinch and zoom in

I managed to track the error down to `setFocusPoint` in `CameraManager`. But like I said, not sure if you'd like to use this as a temporary fix or handle it in another manner.