Make FUSED_PROVIDER SystemApi

This should always have been SystemApi, as most fused providers are
implemented outside system server. There is a strong argument this
should actually be public API, but I'm punting on that for now until we
can give it more consideration.

Bug: 173030969
Test: none
Change-Id: If5ba10accbe2f6ce47d749536d053e1bd8e297f8