Make genrule sandbox script a python script

Unfortunately, genrules are not always available with `m`, instead we
need to know their output paths in order to build them and diff them.
Rewriting in Python lets us store module:output path maps more easily.

Test: ./genrule_sandbox_test.py gen_fstab.gs201 \
      libbt_topshim_bridge_header \
      android-support-multidex-instrumentation-version
Change-Id: If74130e5a4381cc0e1fab396ebb90dfd5a595a1c