Floss: Add Callbacks:get_by_id_mut

Change the default Callbacks:get_by_id to not return mut and
provide the mut version so callers can choose to not use mut
when not needed.

Bug: 193685791
Tag: #floss
Test: emerge-dedede floss
Change-Id: I69839212d649a1e7acc28e54e8e9c9e679856c47