[CLATJ#9] ClatCoordinator: open tun interface

Open the v4-... tun interface for clat. The native function
is moved from netd to jni.

Bug: 212345928
Test: flash and boot
Run "atest ClatCoordinatorTest" in a followup commit.

Change-Id: I3f3c587275125ce3fc7481006947211f17dbb2d4
2 files changed