[CLATJ#1] libclat: move makeChecksumNeutral() from netd to apex

Introduce the library libclat to implement clat functions for jni.
It is helpful for unit test.

makeChecksumNeutral() and its test are moved from ClatdController
without behavior change.

Bug: 212345928
Test: build and boot
      atest libclat_test

Change-Id: I1c0981d98141fc1cc07b2d3a0f3cbddf38683ff3
4 files changed