Fix potential NPE and StorageListener leak

When mStorageManager is null, we must not call its APIs. Also
most of the times, mStorageManager is not null, so onPause()
StorageListener will not get unregistered which can create
leak in StorageListener.

Change-Id: I8b9d2edd7be53036d8d4a5ab8b7e4c4804652e6a
Signed-off-by: Maunik Shah <mshah@codeaurora.org>
Merge "Fix potential NPE and StorageListener leak" into du44