Write relative paths to source_file fields in sdump

This commit adds --root-dir as a parameter to the ABI tools. It defaults
to current working directory. The source_file fields in the dump files
are relative to the root dir, so the file content doesn't depend on the
absolute path to Android source tree.

Test: ./header-checker-unittests
Test: ./test.py
Bug: 154177636
Change-Id: Ia07d0d3b913c921e9e91ea01c4f1e9c3c62c6ea9
179 files changed