MAP: Add content transfer encoding property for MMS body

Usecase:
1) Establish Map Session with Remote Carkit.
2) Send Get Mesage request from remote for MMS message
   with multi mime parts.

Issue:
MMS bMessage content like datetime or sender is not
displayed in valid readable text format.

Root Cause:
CONTENT-TRANSFER-ENCODING bMessage property is not applied
to MMS body part.

Fix:
Add CONTENT-TRANSFER-ENCODING for bMessage body of MMS.

Test: datetime and subject are properly seen on Audi A6 carkit.

Fixes: 70545852
Change-Id: Iee8bb29709e0403e481701e4a49cf1f926f41f96