Improve display name to avoid callback when it did not change

Update the caller display name only if the name actually changed.
This avoids excessive computations on the receiver end as apps such
as dialer listen to these changes and rebuild UI models when they
change.

Defer-CP-To-TM: 240341306
Bug: 240341306
Test: Manual logs to verify callback triggers only when changed.
Test: atest CallDetailsTest

Change-Id: I1c81049240042232ca5c3efa9a75ce2ef2de1829