Setup publishing for Yoga stables

Summary:
This sets up publishing for stable of Yoga whenever a git tag gets published.
I also re-enabled Javadoc publishing as this is a requirement to hit Maven Central.

Reviewed By: NickGerleman

Differential Revision: D46522898

fbshipit-source-id: dc4b9139aed7aff27dce966bcee7b9b0cc4c6fe9

java/build.gradle

@@ -50,6 +50,7 @@ android {
     publishing {
         multipleVariants {
             withSourcesJar()
+            withJavadocJar()
             includeBuildTypeValues('debug', 'release')
         }
     }