Update the avoid bad wifi settings on the handler thread.

Currently, every call to avoidBadWifi fetches the current value
of the config variable and setting whenever it is called. This
means that the score of an unvalidated wifi network can
unpredictably change at any time, creating a data race.

Instead, persist the value and only update it when something
changes.

Bug: 31075769
Change-Id: I0f4e0e742c91ef77fabc95d3ebb494338396aca5