Fix auto-advance on tablet.

Two issues have been fixed:
1. We get the next conversation *before* deleting the existing conversation.
2. Conversation View Fragments are not available in the same manner as
   ConversationListFragment's are. Instead on relying on the existence of a view
   fragment, rely on the conversation view visibility.
3. Next conversation with batch-selected conversations is now correct.

Fix b/6516141

Change-Id: I7a023ff50415347a9cdaf4bd5a9f24fe89f7530e
6 files changed