merge accounting of the picture and dl cache entries in reported statistics

[flar]

We used to report separate statistics for the Picture cache and the DisplayList cache, but really they are somewhat mutually exclusive caches that are used for the same purpose. I've merged our reporting of statistics on those caches for a number of reasons:

• Flutter developers only see the Dart "Picture" class and though we sometimes implement it using an SkPicture and sometimes using a DisplayList, that implementation detail will not be important to them
• Developers are likely familiar with looking for picture cache statistics in various tools and may miss the DL cache statistics or misunderstand why the picture cache statistics would suddenly be different when DisplayList is enabled
• devtools only looks for the picture cache statistics and so is not reporting any statistics for the DL cache

Now both cache statistics will be consolidated and reported under the single "picture cache" set of metrics.