Rename AvoidBadWifiTracker to MultinetworkPolicyTracker

Rename the class and ensure that all code related to bad wifi
avoidance has names that reflect its purpose.  This will allow us
to reuse most of the the code for other purposes in future CLs.

Test: runtest frameworks-net passes
Bug: 34630278
Change-Id: Ida190e5a79e7a12f4eec07e1a3e4d9d138e83bcd