Revamp USB tethering startup flow.

Modify usb tethering flow to no longer require
mUsbTetherRequested. The new simplified flow is:

1. Usb rndis function is set through UsbManager
2. Usb is configured with rndis enabled
3. We tether the USB interface
4. Usb is disconnected
5. We untether the USB interface

This allows USB tethering to be enabled through
either UsbManager or ConnectivityManager and aligns
the flow to be similar to other usb function services
such as MTP.

Bug: 62876645
Test: Enable and disable tethering through connectivity and with `svc
usb setFunction rndis`

Change-Id: Ica4203a499db177999302e099e28948b06ef2298