Convert abi warnings to errors by removing the -advice-only flag from
the invocation of header-abi-diff. This will only apply to branches with
reference abi dumps checked into prebuilts/abi-dumps/(v)ndk (since
abi-diffing will only be triggered on them.)
Also allow for:
1) Extensions.
2) Changes to types in exported headers, unreferenced by public symbols still
trigger warnings.
3) Any removal of symbols appearing in a shared library's .dynsym table, but not
in exported headers is still a warning.
Bug: 64267858
Test: make -j64 from ToT.
Change-Id: Iadeac8d060cb693d4310afb841dc48d91f38597d