Support Android plurals with same name as string

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

```
Monthly

Monthly %2$s
Every %1$d months %2$s

```

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
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