Disable multithreaded dexopt by default

* There is still a race condition here.
* Power users can re-enable this by setting
  persist.sys.dalvik.multithread to true

Change-Id: I03686bafbac1859695466fcb522bc5436ec82e0d
Merge pull request #7 from CyanogenMod/cm-11.0

Disable multithreaded dexopt by default