DexOpt anything in /system on /cache

Allow system property to disable use of /cache for dexfiles.

(manually cherry-picked commit 15d27bbe10e307136e081bedb7cf25131896f1cb)
dvm: Bring back the ANDROID_DATA override

This got broken by the split-cache patch, and it's impacting the
CTS vm unit. Fixit.

Change-Id: I602824afef19c92a4d2c1702d10c8f0315277227