header-checker: Add -allow-adding-removing-weak-symbols

This commit adds `-allow-adding-removing-weak-symbols` option to
`header-abi-diff`.  This allows vendor-specific branch to add or remove
weak symbols from their VNDK shared libs.  This is sometimes necessary
when venders have their compiler toolchain or use different compiler
flags (e.g. inline heuristics).

Bug: 112760591
Test: ./tests/test.py
Change-Id: I94bceacc90cab093e8aa602bb7d866d6f433c5c7
Merged-In: I94bceacc90cab093e8aa602bb7d866d6f433c5c7