`cordova-android` version pinned to 6.3.0

When there are no version specified for the `android` platform in `config.xml`, `cordova platform add` takes the pinned version into account, which is currently 6.3.0 but 6.4.0 of `cordova-android` is available via npm.

```
$ cordova create cordova-platform-test
Creating a new cordova project.
$ cd cordova-platform-test/
$ grep cordova-android config.xml package.json
$ cordova platform add android --verbose
No scripts found for hook "before_platform_add".
No version supplied. Retrieving version from config.xml...
Grabbing pinned version.
Using cordova-fetch for cordova-android@~6.3.0
saving
Running command: npm install cordova-android@~6.3.0 --production --save
Command finished with error code 0: npm install,cordova-android@~6.3.0,-production,-save

...

Prepared android project successfully
No scripts found for hook "after_prepare".
Saving android@6.3.0 into platforms.json
--save flag or autosave detected
Saving android@~6.3.0 into config.xml file ...
adding android to cordova.platforms array in package.json
No scripts found for hook "after_platform_add".
```