base: IPV6 tethering support

Added two APIs addUpstreamV6Interface and removeUpstreamV6Interface,
required to pass the interfaces to use to the Router Advertisement
Proxy.
Added a dun entry to the networkAttributes value in
res/values/config.xml.
This is to allow tethering to work with the dunRequired property.
Added dun to allowable tether upstream type.
Enabled debug flags for debug logging inside framework components.
Regardless of MOBILE_TYPE_DUN APN being available, if dun_required
flag is not set, fall back to HIPRI as a preferred APN for the
embedded data call.
Debug logging was enabled by default which was causing security
vulnerabilities. This patch disables debug logging.

CRs-fixed: 555006
Change-Id: Idb0971449473554bcd749d478563db7ba351d55d
Acked-by: Subash Abhinov Kasiviswanathan <subashab@qti.qualcomm.com>
(cherry picked from commit 43bac130e4bab1c12792d29042f85c6a10372553)
(cherry picked from commit be8491aa9d82177c200eab1af713244a7bbbdcfc)