update_metadata_source doesn't add new entries #172
Description
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.