Revise the options to hide avatars

- Allow to hide avatars on conversation list
- Allow to hide avatars on stock message thread

Change-Id: Ibc597c43ebc125dd5e8323896ffbd2911c64d753

Conflicts:

	res/layout/message_list_item_recv.xml
	res/layout/message_list_item_send.xml

Conflicts:

	res/values-ja/cm_strings.xml