Filter out VENDOR_PYTHONPATH from PYTHONPATH in aosp.

Currently when switching between internal to aosp, PYTHONPATH will be
contaminated. This CL is to ensure PYTHONPATH will be as clean as
opening a new terminal and source/lunch.

Bug: 121106770

Test: internal:
$ source build/envsetup.sh; lunch; echo $PYTHONPATH| sed 's/:/\n/g'
will see VENDOR_PYTHONPATH in PYTHONPATH.

switch to aosp:
$ source build/envsetup.sh; lunch; echo $PYTHONPATH| sed 's/:/\n/g'
won't see VENDOR_PYTHONPATH in PYTHONPATH.

Change-Id: Id1916673000ee3fad55e9611d0c47cc80de86645