Add sources.jar as part of the build artifacts.

Change-Id: Idb5aca9981678da486eef5215f58704123e3fa4e

flexbox/maven.gradle

@@ -50,3 +50,12 @@ install {
         }
     }
 }
+
+task sourcesJar(type: Jar) {
+    from android.sourceSets.main.java.srcDirs
+    classifier = 'sources'
+}
+
+artifacts {
+    archives sourcesJar
+}