Make WifiStaIfaces using the create() method.

Unlike the default constructor, create() populates
the weak_ptr_this_ variable, which is needed for
callbacks from the legacy hal.

Bug: 265857332
Test: Manual test - connect to an AP and walk away
        from it. Check that the RSSI callback is
        triggered in the framework.
Change-Id: I1a10a75c07ea59a3e01b9bb68d9d393bbd20d29d