SystemUI: add USB Tether tile

 * Added vector drawables (blunden)

 * Long click action changed to open Tethering options, in the same way
   as Google does for Hotspot tile (bgcngm)

 * USB Tethering enablement check refactored (bgcngm and luca020400)

Change-Id: Ic44f2b596fe32152f3f722347a855325c52113c6