Pin Java bindings to 1.7

The android_library bazel rule currently enforces Java 7
https://github.com/bazelbuild/bazel/blob/6c1106b1a721516d3b3db54d2e1c31b44a76fbb1/src/main/java/com/google/devtools/build/lib/bazel/rules/android/BazelAndroidSemantics.java#L73

More generally, to enable broader use we pin the source and artifacts
to 1.7 till Java 8 gains wider adoption across Android.