Add diff_flags as a header_abi_checker property

The property allows developers to add extra flags to header-abi-diff for
specific libraries.

Bug: 232891473
Test: m libbinder.vendor
Change-Id: I203af73c784dfc7738bb64f57b224c08d5c595ce
Merged-In: I203af73c784dfc7738bb64f57b224c08d5c595ce
(cherry picked from commit 4f992b66)