Set some GC parameters only for dalvik

Commit CyanogenMod/android_frameworks_base@8f56639
fixed parsing of certain GC values. Before the result was "" and so the
if statement was always false, never executing the setTargetHeap* calls.

Now they are executed but ART currently doesn't support setting
heapminfree and heapconcurrentstart resulting in a UnsatisfiedLinkError
Exception.

Check for the used runtime first and only execute the methods for
Dalvik.

Change-Id: I09e60802685acbec04598646e9f7700f33b53b4c
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>