🐛 Play nice with other plugins including Play Services libraries

[macdonst]

Because of the way cordova-android plugins specify their framework dependencies there is a chance that a plugin will pin an earlier version of play services and it will cause issues for other plugins requiring a later version of play services.

In order to allow end users to specify the version of frameworks in their config.xml file and free up plugin maintainers from having to release a new version of the plugin every time play services updates we introduced variables in the framework tag in cordova 7.1.0.

This PR updates the plugin.xml to use a variable for the play-services-ads dependency. This way users can match the play services versions across all plugins. With this PR merged folks will be able to:

<plugin name="phonegap-plugin-push" spec="2.0.0" source="npm">
  	    <variable name="FCM_VERSION" value="15.0.0" />
</plugin>
<plugin name="cordova-admob-pro" spec="2.0.0" source="npm">
  	    <variable name="PLAY_SERVICES_VERSION" value="15.0.0" />
</plugin>

This way both plugins will compile with play services 15 and not cause any run time errors.

Sorry for the whitespace changes in plugin.xml, Prettier did it.

See: phonegap/phonegap-plugin-push#2360

[astanb]

@floatinghotpot - Please also incorporate this PR into https://github.com/floatinghotpot/cordova-plugin-facebookads, would be massively appreciated!