am e7874c43: Integrate dexpreopt into the build system.

Merge commit 'e7874c430895b9a4e631c89f1e7729da9264d167' into gingerbread-plus-aosp

* commit 'e7874c430895b9a4e631c89f1e7729da9264d167':
  Integrate dexpreopt into the build system.