Support building a par file that does not automatically run

Mainly so that we can build a `python` prebuilt that acts like the
normal python install, though you could also build different ones with
more packages pre-installed.

Bug: 117811537
Test: move built py2-cmd into prebuilts/build-tools/path/linux-x86/python and build
Change-Id: I21215f6fd3754d89f8c65e1dfeb3f2deea23239f