Add empty <application /> element to avoid a Gradle manifest merging error.

Per Android Team (http://stackoverflow.com/a/17757200/937715), library manifests should include an empty `<application />` element.  Otherwise, the application cannot use a targetSdkVersion number that is different than the library when using Gradle/Android Studio.  If the app uses a different targetSdkVersion, you get a compile error `Manifest merging failed. - Main manifest has <uses-sdk android:targetSdkVersion='8'> but library uses targetSdkVersion='19'`.