Prevent PYTHONPATH from accumulatively growing.

After running "lunch" multiple times, the global variable
PYTHONPATH grows larger and larger with repetively content.
This patch is to prevent PYTHONPATH from prepending redundant path.

Bug: b/121110600

Test: $ source build/envsetup.sh && lunch
      $ echo $PYTHONPATH | sed 's/:/\n/g'
      $ lunch; lunch
      $ echo $PYTHONPATH | sed 's/:/\n/g' # not growing.

Change-Id: Ifc36d071fb63bb1494bb0a94fdff477a41708c74