Plugin preference default value is not being saved to <platform>.json

Plugin preference default value is not being saved to <platform>.json

cordova create foo
cd foo
cordova platform add android
cordova plugin add cordova-plugin-ms-azure-mobile-engagement --variable AZME_ANDROID_CONNECTION_STRING=Endpoint=0;AppId=0;SdkKey=0 --variable AZME_ENABLE_PLUGIN_LOG=false

check out `./platforms/android/android.json` or `./pluguns/android.json`
it has the following lines:

"installed_plugins": {
        <...>
        "cordova-plugin-ms-azure-mobile-engagement": {
            "AZME_ENABLE_PLUGIN_LOG": "false",
            "AZME_ANDROID_GOOGLE_PROJECT_NUMBER": "null",
            "AZME_ANDROID_REACH_ICON": "icon",
            "AZME_ANDROID_CONNECTION_STRING": "Endpoint=0;AppId=0;SdkKey=0",
            "PACKAGE_NAME": "io.cordova.hellocordova"
        }
    },

You can see that "AZME_ENABLE_PLUGIN_LOG" is saved here, while "AZME_ENABLE_NATIVE_LOG" is not.

They both can be found in the plugin section of the plugin's `plugin.xml` file:
https://github.com/Azure/azure-mobile-engagement-cordova/blob/master/plugin.xml#L29

Note that the Android-specific preferences have been saved correctly.

cordova@6.3.0 doesn't have this issue.