GC: Improve the string parsing for GC parameter setting

Improve the string parsing for GC parameter system property.
Previously we only recoganize plain number for dalvik.vm.heapminfree
and dalvik.vm.heapconcurrentstart, while users tend to use format as
2m.  Improve the string parse to cover such cases.

Change-Id: Iaf0152bc4d77e1caabc307a9a7b0e5b261f8e441
Set some GC parameters only for dalvik

ART currently doesn't support setting heapminfree and heapconcurrentstart

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