Define paths for new "-exactApi" output files.

This new feature creates files with exactly the annotated APIs
instead of a giant superset.  (This makes it easier to compare diffs
between releases.)

Test: make update-api && make -j32 online-system-api-sdk-docs
Bug: 38146029
Change-Id: Ia7ddac43a0ead380025e248ee22072e11c50e263