Embed R8/ProGuard rules for native methods

[JakeWharton]

• Embed rules
  • Add shrinker rules to keep native names #590
• Test with R8
  • Add R8 test run #669
• Test with ProGuard
  • Test with ProGuard #591
• Automatically extract dependency rules (and our own rules) from the jars.
  • R8
    • Add R8 test run #669
  • ProGuard

[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.