Add share library `libapfv6` and `libapfbuf`

Separate the test_buf_allocator.c/h to individual `libapfbuf`.
This will be used for `libapfv6jni` in the follow-up CL.

Bug: 391118179
Test: atest NetworkStackTests
Change-Id: Ifbf9490d5152a22df8bee8d5a789bbf160557d70
3 files changed