[SPARK-2307] SparkUI - storage tab displays incorrect RDDs

The issue here is that the `StorageTab` listens for updates from the `StorageStatusListener`, but when a block is kicked out of the cache, `StorageStatusListener` removes it from its list. Thus, there is no way for the `StorageTab` to know whether a block has been dropped.

This issue was introduced in #1080, which was itself a bug fix. Here we revert that PR and offer a different fix for the original bug (SPARK-2144).

Author: Andrew Or <andrewor14@gmail.com>

Closes #1249 from andrewor14/storage-ui-fix and squashes the following commits:

af019ce [Andrew Or] Fix SPARK-2307