We want to know what objects had bitmaps in the walk of the
request. We can check their position in the history and evaluate our
bitmap selection algorithm.
Introduce a listener interface to the BitmapIndex to report which
getBitmap() calls returned a bitmap (or not) and a method to the
bitmap index to set the listener.
Change-Id: Iac8fcc1539ddd2dd450e8a1cf5a5b1089679c378
aab75dba7e