Don't notify on network switch if source network is validated.

Sometimes we switch away from a network to another (e.g., wifi to
cell data) not because the old network is unvalidated, but
because the score is lowered by a low signal strength.

In this case, don't notify the user of a network switch.

Bug: 31132499
Change-Id: I996a6e00096f8cb864fa9b00b36921725a4edb53