Refactor libposepredictor to be self-contained.

The CL seems big but it is mainly moving stuff around to
make libposepredictor self contained.

libposepredictor/include/predictor.h exposes all functionality and
it only includes Eigen stuff.

Also adds a shared library build step to the makefile.

Bug: 35362498
Test: No functional change.
Change-Id: I0c850711ffde16ee75e4c88b09e47f761c100385