email: always refresh actionbar menu on select/deselect conversations

The read/unread status of the conversations must be update to reflect the current selection.

Use case:

1.- On a folder with 2 or more messages, select on of them and mark it as unread
2.- Select other conversation which status read
3.- Deselect the unread conversation

The actionbar still reflect the mark as read action, but only one conversation with
a read status is selected.

Change-Id: I86dac602dc8f396555dbc1e3327a40206090cc30
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
UnifiedEmail: Improve string

Change-Id: Ida9bb8d98da10612b0110f9b6d0a36bf6a9efaa6