Change par file generate rules.

Make sure the shebang of the par file is consistent with the main python
script rather than use /usr/bin/python arbitrarily.

Bug: 185399398
Bug: 185094069
Test: m atest ; atest-dev --help
      m atest-py2 ; atest-py2-dev --help
Change-Id: Ice4590b3d173e6d11e608c2ca29b297df0dc4089