Support "don't ask again" in the avoid bad wifi dialog.

This contains the following changes:

1. Make NETWORK_AVOID_BAD_WIFI a tristate: 0 means never avoid
   bad wifi, unset means prompt the user, 1 means always avoid.
2. Look at NETWORK_AVOID_BAD_WIFI only if the carrier restricts
   avoiding bad wifi (previously, we relied on the setting being
   null and defaulting to the value of the config variable).
3. Add an avoidUnvalidated bit to NetworkAgentInfo to track
   whether the user has requested switching away from this
   unvalidated network even though avoiding bad wifi is generally
   disabled. This is set to true when the user selects "switch"
   in the dialog without setting the "Don't ask again" checkbox.
4. Add a hidden setAvoidUnvalidated API to ConnectivityManager to
   set the avoidUnvalidated bit.
5. Additional unit test coverage.

Bug: 31075769
Change-Id: I1be60c3016c8095df3c4752330149ce638bd0ce1