Install abseil headers to cmake shared library build

 * Since commit 5f004516 tensorflow::StringPiece is replaced by
   absl::string_view, so abseil headers should be installed to shared
   library build to fix compilation error for out-of-source build.
 * To cleanly copy abseil headers, disable in source build for abseil to
   avoid src tree been polluted by cmake generated files.
 * Meanwhile, remove _build suffix from abseil_cpp product name since
   it's confusing.