Embed R8/ProGuard rules for native methods

[JakeWharton]

• [x] Embed rules
  • #590
• [x] Test with R8
  • #669
• [x] Test with ProGuard
  • #591
• Automatically extract dependency rules (and our own rules) from the jars.
  • [x] R8
    • #669
  • [ ] ProGuard
    • https://issuetracker.google.com/issues/394478509

[JakeWharton]

Would be nice to add a JVM test variant which runs R8

[JakeWharton]

This is a huge pain in the ass.

R8 problems:

• It does not filter out the contents of folders like it does jars, so the JNI libraries offend it. I would have to create a jar solely for it to immediately un-jar and apply the correct filtering rules.
• It does not read embedded shrinker rules, neither in folders nor jars. WTF?

I didn't even get to running ProGuard yet...

[JakeWharton]

ProGuard produces a VerifyError. Sigh.