Remove unused resources from release builds

...using "Resource shrinking".  From a quick test, this cuts the OBA Google release APK from 3.334 MB to 2.788 MB.  This becomes more important as we're using more and more support libraries, especially the Android Design Support library, which are going to have a lot of embedded resources, many of which we won't use.

onebusaway-android/build.gradle

@@ -222,6 +222,7 @@ android {
         release {
             signingConfig signingConfigs.release
             minifyEnabled true
+            shrinkResources true
             proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-project.txt'
         }
     }