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: Ib9895534a4b334ce571af433c97d1eefa97a249a