If user ejects a storage from details, take user to...

... the storage list view

1. Change to ExternalStorageProvider
When available volumes have changed, it now notifies on
"content://com.android.externalstorage.documents" rather than on
"content://com.android.externalstorage.documents/root/", because it'll also
affect "content://com.android.externalstorage.documents/document/*/children".

2. Change to DocumentUI
- DirectoryLoader now won't crash when a provider returns null.
- DirectoryFragment now closes itself (i.e. emulate a back press) when a load
failed.
- It now correctly reloads contents when a volume is ejected thanks to 1.

Bug 21472170

Change-Id: Icf4bbc2ebb9067313dbbb67083cd4115b0a75c58