Avoid android-support-v4.jar errors

If there is a plugin already installed which uses `android-support-v4.jar`, when cordova tries to copy the library with `<source-file src="android-support-v4.jar" target-dir="libs" />` it fails with error `android-support-v4.jar" already exists!`.

How to reproduce: install com.admob.google plugin from https://github.com/appfeel/admob-google-cordova first and then the android.support.v4 plugin:

```
cordova plugin add com.admob.google
cordova plugin add https://github.com/MobileChromeApps/cordova-plugin-android-support-v4
```