frameworks/base: IPV6 tethering support

This patch adds two new API calls addUpstreamV6Interface and
removeUpstreamV6Interface to the tethering framework to pass the IPv6
upstream and downstream interfaces to netd if IPv6 is available.
Required to enable ICMPv6 forwarding.

[*]IPV6 tethering: Check info for null before removeUpstreamV6Interface

When tethering on dual stack networks such as verizon wireless and
sending or recieving MMS you get an null pointer exception and hard
reboot.

It starts to go wrong here:
10-14 19:11:51.375: E/Mms/smil(28035): Failed to parse SMIL document.
10-14 19:11:51.375: E/Mms/smil(28035): org.xml.sax.SAXParseException:
expected start tag notEND_DOCUMENT null@1:1 in java.io.InputStreamReader
@4192ada0

And then:
10-14 19:11:55.559: E/AndroidRuntime(23125): java.lang.NullPointerException
10-14 19:11:55.559: E/AndroidRuntime(23125):    at com.android.server.
connectivity.Tethering$TetherMasterSM$TetherModeAliveState.processMessage
(Tethering.java:1573)

Which is line 1573 of:

https://github.com/CyanogenMod/android_frameworks_base/blob/cm-10.2/services/java/com/android/server/connectivity/Tethering.java#L1573

CMD_UPSTREAM_CHANGED's message.obj can be null (line 776).

Change-Id: I1f4f6284b2f152d8b4f0fbd02ac5a17f57c71dd7
Merge "frameworks/base: IPV6 tethering support" into kitkat