Currently, when the app_voicemail sending VoicemailUserEntry AMI event, there's
no OldMessageCount info for default.
To check the OldMessageCount info, it required IMAP_STORAGE define, but this is
not correct.
Added OldMessageCount item as a default.

ASTERISK-27456

Change-Id: I5c71521c2d1daf8b7b161e31c34d28cca6aea4c7