MAP: Add defaults and fix sender_name

- Correct senderName parameter attribute name to "sender_name".
  Add length check to limit the characters  allowed for this
  attribute value to 256, as per the Spec.
- Set default SMS type to SMS_GSM.
- Set default subject, address values to empty for extended
  MNS event report to handle cases for null valued fetched from db.

Test: MAP functionality works fine.

Bug: 35590443
Change-Id: I3dc787dbf420dbab030713c035a16d935ab64149