soong_ui: Build under a path that contains a symbolic link.

The build directory passed in to soong_ui can contain a
symbolic link. soong_ui was not evaluating the build
directory to retrieve the true path, hence failing to
execute the internal soong.

Fixes: b/135995632
Test: Unit test case and executed the scenario in the bug
      description.

Change-Id: I5779c6aa3f3183810437dbe2b2d4e40acbafb205