header-checker: Add config file support

This commit add config file support to header-abi-diff so that we can
have per-branch configuration.

Bug: 112760591
Test: ./tests/test.py
Test: ${ANDROID_HOST_OUT}/nativetest64/header-checker-unittests/\
      header-checker-unittests
Change-Id: I1fbd76c93991a2b36121a5f4397c62eb72d486ec
Merged-In: I1fbd76c93991a2b36121a5f4397c62eb72d486ec