art: Implement SetTargetMinHeapFree, SetTargetHeapConcurrentStart

* Commit: GC triggering performance optimizations
  ID: 7646e2eb41024d152d76b269653c2321e2cfbd23

  introduced additional functions that needed to be implemented.

Change-Id: I5ec4ae74dc70f1859db2bae8588e697b147a2682
Merge branch 'kitkat' of github.com:AOSPA-legacy/android_art into kk4.4

Conflicts:
	runtime/gc/heap.cc
	runtime/gc/heap.h
	runtime/native/dalvik_system_VMRuntime.cc

Change-Id: Ifd9e5a091e0b1aa204ee54f90ccd49f45c16b6df