ipconnectivity.proto: change Transport to LinkLayer

... to clarify ambiguities about the purpose and goal of the
LinkLayer/Transport enum.

This renaming is safe because at the moment there is no reader or
writer of this field anywhere in google3 or android.

Test: $ runtest frameworks-net
Change-Id: I6775cc2df3438d3bd8609a86ed1f4b4bf6296816