Fix sender name string to match MAP spec

In section 3.1.6 of the Bluetooth MAP spec version
1.2.2, the field name should be "sender_name" instead
of "senderName".

Bug: 26904410
Change-Id: I0dacaf50b0830239d2c2e7e787dc7998f3ed6723
(cherry picked from commit 9a18a9fc)