Add dalvik.vm.gctype to enable switching between different GCs.

The option passes the specified GC type into the Xgc runtime
option.

Currently the three different supported GC types are MS, CMS, SS
which represent mark-sweep, concurrent mark-sweep, mark-sweep +
semi-space hybrid.

Change-Id: I6ad95c8d12c0d1158f7c861ff0c3180761619172