Up navigation

1. Added `launchMode="singleTask"` on `ConversationActivity` in the manifest. This makes the default up behavior less glitchy.
2. Changed the intent `ManageAccountActivity` sends to `ContactsActivity` when there are no conversations.
3. Removed `FLAG_ACTIVITY_TASK_ON_HOME` from the intent sent by `ShareWithActivity`. This has the effect that the back button can return you to the application that initiated the share.