Fix USB tethering

The previous approach no longer works with the new USB drivers, since the usb0
interface is no longer enabled by default.
This introduced a chicken & egg problem - usb0 will not be enabled until the
user tries to start tethering, but Settings will not enable the checkbox unless
usb0 is enabled.
To fix this we add an explicit call to start USB tethering in the connectivity manager.
This will enable RNDIS if necessary and then bring up tethering once usb0 is enabled.

Change-Id: If3972c89e05377af00b7a71cc2588e44bd4cfaed
Signed-off-by: Mike Lockwood <lockwood@android.com>