Create overlaid directory if and only if the target has overlays

Rather than creating an overlaid directory by default
on all busytown builds, create an overlaid directory for all
targets that have overlays. This has several consequences:

* Targets that use overlays will always construct
the overlaid source in a temp directory, regardelss of
weather you're using container.py or build_busytown.py.
* Targets that don't have overlays will never use a temp
overlaid source directory.
* Only nsjail.py and overlay.py will have knowledge of
overlays.

Test: python build_busytown_test.py \
    && python container_test.py && python nsjail_test.py && \
    python overlay_test.py
Bug: 122848229
Change-Id: I11bd98fbd403dc8898e62eb83142851dfb5cf180