art: Implement SetTargetMinHeapFree, SetTargetHeapConcurrentStart

* Commit: GC triggering performance optimizations
  ID: 7646e2eb41024d152d76b269653c2321e2cfbd23

  introduced additional functions that needed to be implemented.

Change-Id: I5ec4ae74dc70f1859db2bae8588e697b147a2682
(cherry picked from commit 1ca6a11a61b57edea370adf4454876ccf869d92e)