runtime: Add support for dexroot-on-cache

If installd puts the dex files there, we want to read/write our oat
files out of there, too

Change-Id: I7da528622d7c855431dd99b20ff54da791bc1681
runtime: dexroot-on-cache: obey dalvik.vm.dexopt-data-only

Change-Id: Ie037706b1004e014c20f81c8fcdc219c46da57c7
tools: use '/usr/bin/env python' instead of '/usr/bin/python'

* This is how it is done in other scripts in the AOSP tree, as
  '/usr/bin/python' may be python3 on some distros, which isn't
  supported yet.

Change-Id: I0d9857fc6a122f505953fddcd6244dad75b1838a