Add the GoogleServices preference since cordova-android supports it

wrt
e-mission/e-mission-docs#554 (comment)

the native instructions suggest adding it to build.gradle
So I looked at `build.gradle` and found

```
if (cdvHelpers.getConfigPreference('GradlePluginGoogleServicesEnabled', 'false').toBoolean()) {
    apply plugin: 'com.google.gms.google-services'
}
```

That just clarified that we don't have to wait for the plugins to edit the config file,
we can just edit the config file directly.

After adding the preferences, I don't get the push notification error on startup, AND

```
platforms/android/app//build/generated/res/google-services/debug/values/values.xml
```

has the appropriate value.

This should be the last pending issue

config.cordovabuild.xml

@@ -16,6 +16,8 @@
     <preference name="android-minSdkVersion" value="22" />
     <preference name="BackupWebStorage" value="none" />
     <preference name="emSensorDataCollectionProtocolApprovalDate" value="2016-07-14" />
+    <preference name="GradlePluginGoogleServicesEnabled" value="true" />
+    <preference name="GradlePluginGoogleServicesVersion" value="4.3.3" />
     <config-file parent="/manifest/application" target="AndroidManifest.xml">
         <meta-data android:name="firebase_analytics_collection_deactivated" android:value="true" />
         <meta-data android:name="google_analytics_adid_collection_enabled" android:value="false" />