Support Android plurals with same name as string

[nijel]

It's perfectly valid to have string with same name as plural, for example:

    <string name="monthly">Monthly</string>
    <plurals name="monthly">
        <item quantity="one">Monthly %2$s</item>
        <item quantity="other">Every %1$d months %2$s</item>
    </plurals>

However Weblate currently uses only name to identify the strings, so they appear as duplicate ones to Weblate.

To fix this, we probably have to construct internal ID for these strings differently. This change will cause problems to existing translations as they will have to be imported again to get new IDs...

See http://developer.android.com/guide/topics/resources/string-resource.html