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
(cherry picked from commit 34ee56fe6fda8fcb2572947a4d2e6bf77e0b8899)