Add proguard rules

[alanlee-nd]

Problem to be addressed

I did find an old issue with the same topic: https://github.com/natario1/CameraView/issues/163#issuecomment-372818221

But it would be ideal if the library owner provides progaurd rules so the library can be used in it's optimized binary form instead of having the app developer figuring out the rules. Most Android library I know of provide their own rules.

Describe the solution you'd like

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Additional context

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