Clear selection set when viewing messages on one-pane

This is missing one crucial functionality:
 In conversation view, if we perform destructive actions on the selected conversations, the
 selected set is not accurately updated.

For a correct implementation, we need a ConversationPositionTracker class, so that will be solved
in a second pass.

Change-Id: Ie9909ce026c4ec1148b4817d6968699b392e2330
4 files changed