Change conditionallyCheckInstanceCounts to use countInstancesOfClasses

countInstancesOfClasses allows passing multiple classes unlike
countInstanceOfClass. This provides a speedup since we dont traverse
the heap multiple times.

Changed ActivityThread.dumpMemInfo to do a GC since countInstancesOfClass
no longer does GC before counting.

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

Change-Id: Ia85684f40cf59a52aa71a8479c711a994651209b