Migrate from Gradle to Maven.

- Restructure source to src/{main,test} style
- Add pom.xml and update Android.mk
- Migrate all tests to JUnit4 and Robolectric
 - RequestQueueTest is currently @Ignored as fixing it will
   involve more extensive refactoring.
- Main library still builds in Gradle; tests do not

Change-Id: I1edc53bb1a54f64d3e806e4572901295ef63e2ca
diff --git a/build.gradle b/build.gradle
index 7c0abdb..1a864dc 100644
--- a/build.gradle
+++ b/build.gradle
@@ -11,7 +11,6 @@
 
 android {
     compileSdkVersion 19
-    buildToolsVersion = '19.1.0'
+    buildToolsVersion = '21.1.0'
 }
 
-apply from: 'rules.gradle'