Address the TODO to allow tagging sockets, which was done before the
code was imported to the Android codebase.
Also remove the comment in MdnsSocket, as the tagging is already done by
setting the thread tag, and tagSocket in the original code actually did
nothing for DatagramSockets.
The option is unset in the Android repository, so this code is unused
there.
Test: atest
Flag: EXEMPT unused in Android repo
Change-Id: Ifccc4ec2f0b63bf2caabe1d9ef4f57f08731a2b1
766451a785