apilint: Fix API lint issues 2/2

Fixes false positives that occur when a class in current.txt is faulty, and an
entry for that class is then added to system-current.txt.

This was so because when parsing the previous revison's system-current.txt, we
did not know about the class and thus didn't look for it in current.txt, and
thus never recorded that the error is preexisting.

To avoid that, we track all classes in system-current.txt with a matching entry
in current.txt in the current revision, and later use that to look up all classes we
may have missed when examining the previous revision.

Test: python tools/apilint/apilint_test.py
Change-Id: Ibe09f1159e351e56b35b8816ce0db760de4ef791