Bug 1667675 - Use sourceSets instead of JavaCompileProvider to generate sources.jar. r=agi

Documentation is lacking but apparently
`variant.javaCompileProvider.get().source` is always empty according to
da34aabdb6.

In that commit, `android.sourceSets.main.java.srcDirs` was the
replacement. However, I used
`variant.sourceSets.collect({ it.java.srcDirs }).flatten()` to be
consistent with an earlier usage in `build.gradle`.

Differential Revision: https://phabricator.services.mozilla.com/D91836

mobile/android/geckoview/build.gradle

@@ -348,7 +348,7 @@ android.libraryVariants.all { variant ->
         classifier 'sources'
         description = "Generate Javadoc for build variant $name"
         destinationDir = new File(destinationDir, variant.baseName)
-        from files(variant.javaCompileProvider.get().source)
+        from files(variant.sourceSets.collect({ it.java.srcDirs }).flatten())
     }
 
     task("checkstyle${name.capitalize()}", type: Checkstyle) {