Rework issues with downloadable voices and network availibility changes

[lumpidu]

• do network connection check before downloading a voice
• make sure, the device voice inside the description for downloaded voices really is unique
• add equals() and hashCode() methods to DeviceVoiceXXX Pojos for uniqueness comparison
• delete voice info from downloaded voice description, there were dangling voice info descriptions inside
• synchronize access to public methods in DownloadVoiceManager
• proper error handling in async download thread by calling appropriate observer callbacks
• don't subscribe multiple times the same kind of observer for changes of voice db
• bugfixes in case network connection comes back and pressing "download"
• don't log a load of progress for "every byte" downloaded, but only each 10% iteration
• make sure to run on UIThread when changing mProgressBar