Make sure that cordova is injected only once on android

[pgaubatz]

This fixes #15

[tisdall]

I'm unfamiliar with Android development, but is there any way to detect that the webview is crosswalk before bothering to search for the script in the DOM?

[pgaubatz]

Hi @tisdall
not that I'm aware of.
But what exactly is your concern?

[tisdall]

Well, the issue is only applicable to Crosswalk, so if there was a quick way to detect Crosswalk before searching through the DOM then it'd probably be faster. This is really an issue in Crosswalk so it'd probably be better to encapsulate it in a "if crosswalk then" type statement.