API Lint: Add support for base current.txt

Allows specifying a base current.txt and previous.txt file when linting
system-current.txt and test-current.txt to avoid false positive error
messages due to public API members not being duplicated in the respective
non-public APIs

Test: python tools/apilint/apilint.py --base-current=api/current.txt api/system-current.txt
Change-Id: I306a99b1423584ef3fcdc9272a83cb5eacc37227