Start building SystemApi docs again; exact output.

Now that we've fixed the docs build we can enable it again.  Also
start using the new "-exactApi" feature to get a file 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: Ib6d5d931f71ac4f50bf3223c25b50ed5765406a3