Expose a callback when no cameras are detected.

[mkotyk]

Instead of throwing a Runtime exception, this patch exposes a callback to indicate there were no cameras enumerated. Tested using emulator.

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[mkotyk]

Trov signed it!

[phileo]

Looks good, the only addition I would make is to call mCallback.onCameraNotAvailable() instead of barfing when the exception is thrown in Camera2.startOpeningCamera()

[googlebot]

CLAs look good, thanks!