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

Conflicts:

	core/java/android/app/ActivityThread.java