Support non-standard path of Android Support Repository

Summary:This is needed in cases when the Android Support Repository is on a non-standard path.

**Test plan (required)**

CircleCI tests on this pull request.
Closes https://github.com/facebook/react-native/pull/7096

Differential Revision: D3207267

Pulled By: mkonicek

fb-gh-sync-id: 7539407f485341352faadc3b07d2c1459e0527f5
fbshipit-source-id: 7539407f485341352faadc3b07d2c1459e0527f5

build.gradle

@@ -18,5 +18,10 @@ allprojects {
     repositories {
         jcenter()
         mavenLocal()
+
+        def androidSdk = System.getenv("ANDROID_SDK")
+        maven {
+            url "$androidSdk/extras/m2repository/"
+        }
     }
 }