[CLATJ#10] ClatCoordinator: detect mtu

Detect the available mtu for clat. The native mtu detect function
is moved from netd to libclat which is called by jni. The java
adjust mtu function is moved from netd native code.

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

Change-Id: Icf5f57c7e7b947997b76152722e46acdd9d95c1a
2 files changed