[Sb refactor] flag for deprecation of SignalCallback

Everything that moves clients off of the old SignalCallback and onto the
new pipeline can go behind this flag. This flag should be deleteable
once NetworkController is also deletable.

Test: compile
Bug: 291321279
Change-Id: Ia5c8054be0010eef4c50a21745b427ce3746ea2a
1 file changed