Enhance the merge of the native configuration files

[valentinstoychev]

There are two things we can do better with the merge of the plugin config for the native projects

• Check if there are duplicate entries of the resulting merged configuration file and leave only one.
• Merge not only on the root level, but also scan the sections and apply the changes in the configuration sections.

For example when you have this config snippet in the plugin AndroidManifest.xml:

<application>
   <meta-data android:name="com.google.android.gms.version"
    android:value="@integer/google_play_services_version" />

   <meta-data android:name="com.google.android.geo.API_KEY" android:value="AIzaSyCMjyff-PWVM9N5KCSZmXngbwZjBPZB-E8"/>
</application>

we should make sure that these declarations are merged into the existing <application> section of the native AndroidManifest.xml and not to create a second <application> section, which is later ignored by the Android runtime.

[PanayotCankov]

I think we should allow the API key to be added to source control. node_modules are usually ignored from source control so the module files won't be suitable.

[ligaz]

This has been addressed with our Gradle support: #687