Fail if the interface is not available when starting

Addresses a long-standing TODO.  Now, when calling IpClient's
startProvisioning(), the interface has to be available (i.e.
InterfaceParams#getByName() must return non-null).

Also:
    - add a test
    - refactor for testability
    - delete some constructors no longer used
    - properly handle passed-in null IpClient.Callback
    - some more IpManager -> IpClient renaming
    - permit recording metrics before starting a provisioning
      attempt (logging immediate errors) without Log.wtf().

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks/opt/net/wifi/tests/wifitests/runtests.sh passes
    - runtest frameworks-net passes
    - basic WiFi IpClient connections works fine
Bug: 62476366
Bug: 73487570
Change-Id: Ic83ad2a65637277dcb273feb27b2d1bb7a11eb2b