Add PorterDuff mode prop

[andre-krueger]

Summary

This should fix #78.
As masking an image with an alpha mask image didn't work on Android, I added a prop to the MaskedView to set the PorterDuff mode on Android. With SRC_OUT I could successfully mask an image with another image.

Test Plan

What's required for testing (prerequisites)?

Insert an image into the maskElement prop and inside MaskView insert an alpha mask image. Set the porterDuffMode prop to SRC_OUT.

What are the steps to reproduce (after prerequisites)?

The alpha mask image should be correctly applied to the image.

Compatibility

OS	Implemented
iOS	❌ (already works on iOS)
Android	✅

Checklist

• I have tested this on a device and a simulator
• I added the documentation in README.md
• I mentioned this change in CHANGELOG.md
• I updated the typed files (TS and Flow)
• I added a sample use of the API in the example project (example/App.js)

[albertovasquez]

This does not seem to fix the issue