Update comment related to unread messages MAP

Fix comment in processMessage() : case MSG_GET_MESSAGE_LISTING to
reflect the code implementation. We request the latest 50 messages, not
just the latest 50 "unread" messages.

Tag: #stability
Bug: 338079847
Flag: EXEMPT, comment change only
Test: atest MapClientStateMachineTest.java
Change-Id: I8114b1b277efb394fbc38f80ea2317f3c620eb93