fix crash (Issue #11)

[phileo]

PreviewImpl.Callback.onSurfaceChanged() assumes that the camera has been initialized and started properly. However, there is no check for this when startCaptureSession() is called by onSurfaceChanged().
In particular, on first launch after install, the OS permissions dialog appears, which prevents startCaptureSession() from starting normally (from Camera2.start() ). This invalidates the original assumption. However, startCaptureSession() is called anyways, which leads to the IllegalStateException due to the camera not being started properly.

The fix is to guarantee that startCaptureSession() is not called prematurely until the camera has been properly started.

This fixes issue #11

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