`update_metadata_source` doesn't add new entries

[loremattei]

The android_update_metadata_source and ios_update_metadata_source can only update existing entries, but they don't add new entries to the file even if the entries are correctly defined in the parameters.

In order to add a new string, the user shall manually add it to the .po file. After that, release-toolkit is able to update them automatically.

We should fix the above actions so that they add new entries automatically when they are added to the parameters.