Switch to USB_CONFIGURED as the primary USB readiness signal

Test: as follows
    - built
    - flashed
    - booted
    - "runtest frameworks-net" passes
    - USB tethering on and off works as expected
Bug: 32163131
Bug: 36216864
Bug: 62147658
Bug: 62552150
Change-Id: Ia8f7f3616f1358b0427386ce8aff26899e03ac07