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 9a18a9fc4349f90e49d036ccafc91d8b5befe973)
diff --git a/src/com/android/bluetooth/map/BluetoothMapContentObserver.java b/src/com/android/bluetooth/map/BluetoothMapContentObserver.java
index ab91ac7..a68298a 100644
--- a/src/com/android/bluetooth/map/BluetoothMapContentObserver.java
+++ b/src/com/android/bluetooth/map/BluetoothMapContentObserver.java
@@ -768,7 +768,7 @@
                             subject.substring(0,subject.length() < 256 ? subject.length() : 256));
                 }
                 if (senderName != null) {
-                    xmlEvtReport.attribute("", "senderName", senderName);
+                    xmlEvtReport.attribute("", "sender_name", senderName);
                 }
                 if (priority != null) {
                     xmlEvtReport.attribute("", "priority", priority);