Allow to use ncm function for usb tethering

Adding the configuration to decide using rndis or ncm for usb tethering.
If ncm is configured for TETHEIRNG_USB, then TETHERING_NCM is not
available.

Bug: 162920185
Test: atest TetheringTests
      manul testing usb tethering
Original-Change: https://android-review.googlesource.com/1730541
Merged-In: Ifc7eee2457a950a0e2d1c3cc89a3373a7ad23c9c
Change-Id: Ifc7eee2457a950a0e2d1c3cc89a3373a7ad23c9c
diff --git a/Tethering/res/values/config.xml b/Tethering/res/values/config.xml
index 4391006..0412a49 100644
--- a/Tethering/res/values/config.xml
+++ b/Tethering/res/values/config.xml
@@ -193,4 +193,8 @@
          state. -->
     <!-- Config for showing upstream roaming notification. -->
     <bool name="config_upstream_roaming_notification">false</bool>
+
+    <!-- Which USB function should be enabled when TETHERING_USB is requested. 0: RNDIS, 1: NCM.
+         -->
+    <integer translatable="false" name="config_tether_usb_functions">0</integer>
 </resources>