Make package compatible with iOS projects

[miguelsm]

When working on platform independent projects, i.e. that support both Android and iOS, and we may want to have conditional imports like this:

const SharedPreferences = Platform.OS === 'android' ? require('react-native-android-shared-preferences') : null;

RN is not able to find this module when bundling for iOS because it uses index.android.js as the main file in package.json. Renaming that file to index.js would prevent this issue.