hoststubgen: Emit stats for dashboarding.

As we expand our audience, developers will be interested in knowing
what APIs are supported through a top-down dashboard view that we
can continually update over time.

This change emits a statistics CSV that can be easily bulk-imported
to generate a dashboard.

Bug: 322895594
Test: TH
Change-Id: Idea55b64cdb79e9a49f63340f83a1b395f8e5ec7