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
ART: Use python as defined in env to fix build on some systems
Ensures the correct python version is used to prevent syntax errors

Change-Id: Ie57cf024341784284c2d2c5625c825f8b4544881