Move the packet socket initialization from netd to jni because
mainline module is going to launch clatd. Need to provide packet
socket for reading local 464xlat IPv6 packets.
Bug: 212345928
Test: flash and boot
Run "atest ClatCoordinatorTest" in a followup commit.
Change-Id: I1cef6e099403f883d668081fb5e917863f5a3c2a