Moving junit out of core.jar and into core-junit.jar

This commit was submitted too early:
   https://android-git.corp.google.com/g/50610
...and then rolled back:
   https://android-git.corp.google.com/g/50622

Change-Id: I5f4e53bd0c892ab64711b99b2e95d2f101626dbc
Merge (-sours) commit 'f2f3e4b234271a20ff3663256081c8aed26f7807' into oxygen_4.3

* commit 'f2f3e4b234271a20ff3663256081c8aed26f7807':
  Moving junit out of core.jar and into core-junit.jar