Implementation of MulticastChannel.

Note: ProtocolFamily / StandardProtocolFamily /
DatagramChannel.open(ProtocolFamily) have not been
implemented.

There is a related change to libnativehelper that
must be merged at the same time to avoid
build breakage. See
https://android-review.googlesource.com/#/c/81371/

Bug: 12464155
Change-Id: I07fc049b429a2d373e9bd7b07149632f5cd69f9d
23 files changed