Fix issue #2163654: deadlock, runtime restart

Don't hold a lock when the activity thread is telling the activity manager
to release a provider.

This requires that the activity manager now keep a reference count on the
providers, because without the lock it is possible for activity thread to
call in to request the provider again before it has finished telling
about the release.

Change-Id: I5f912903891f4edae85e28819d4e6f14b8f2e688