UsbGadget AIDL migration cleanup

This CL does the following:
  - Add fields to AIDL UsbSpeed that were added in HIDL's UsbSpeed 1.2.
  - Fix the values of the AIDL speeds. The values of fields in UsbSpeed
    didn't match their HIDL counterparts so this CL fixes the values
    as well.
  - Remove unused import from IUsbGadget and IUsbGadgetCallback.

Test: Manually tested that UsbManager reports the correct speed when
      using the AIDL interface
Bug: 218791946
Change-Id: Ic94bb382c11a88a29da5e61ca751138f271f54ac