tombstoned: Add a shared library version of libtombstoned_client...

.. for ART and the frameworks to link against. In the new stack dumping
scheme (see related bug), the Java runtime will communicate with
tombstoned in order to obtain a FD to which it can write its traces.

Also move things around to separate headers that are private
implementation details from headers that constitute the public debuggerd
API. There are currently only three such headers :

- tombstoned/tombstoned.h
- debuggerd/client.h
- debuggerd/handler.h

Bug: 32064548
Test: make

Change-Id: If1b8578550e373d84828b180bbe585f1088d1aa3