Add VMDebug.countInstancesOfClasses

countInstancesOfClasses allows passing multiple classes unlike
countInstanceOfClass instead of needing to do one at a time.

This is going to be used to speed up string mode checking.

Also changed the logic to not do a GC, this was the old Dalvik
behavior. It is the job of the caller to do this.

Added test.

https://code.google.com/p/android/issues/detail?id=177552

Change-Id: Ia85684f40cf59a52aa71a8479c711a994651209b