Used exported flatbuffer generated includes cmake property.
Used exported flatbuffer generated includes cmake property.

Previously flatbuffer generated includes were being exported from
the project using the PARENT_SCOPE parameter of cmake set().  Using
PARENT_SCOPE has the drawback that *all* files that include a target
using add_subdirectory() must export all variables for those
variables to be visible in a parent project.

Instead this uses the ${project}_DIR property of the
${project}_generated_includes target to get the generated includes
directory which is globally available.

Tested:
Verified everything still builds on Linux.

Bug: 25185745
Change-Id: I8e8ab9db7c3d210065a3eccdaec7da49b2be7f81

google/scene_lab,google/scene_lab