Bring back date format and timestamp of crash reports to be always identical no matter what locale is set

This was introduced in https://github.com/bitstadium/HockeySDK-iOS/commit/4dbbef38e2ec008627b1f79ba10a044ec9a2495c but when merging PLCR changes back in it accidentally got removed
Make sure that BITHockeyLocalizedString always returns a valid string
Safe app version usage time in its own property list file instead of the user defaults