[android] Use a nested RecyclerView with a single items to show image attachments

...oject/briar/android/conversation/ConversationAdapter.java

...riar/android/conversation/ConversationItemViewHolder.java

...r/android/conversation/ConversationMessageViewHolder.java

...ar/android/conversation/ConversationNoticeViewHolder.java

...r/android/conversation/ConversationRequestViewHolder.java

...briarproject/briar/android/conversation/ImageAdapter.java

...arproject/briar/android/conversation/ImageViewHolder.java

...ect/briar/android/conversation/SingleImageViewHolder.java

.../src/main/res/layout/list_item_conversation_msg_image.xml

...main/res/layout/list_item_conversation_msg_image_text.xml

...oid/src/main/res/layout/list_item_conversation_msg_in.xml

...id/src/main/res/layout/list_item_conversation_msg_out.xml

briar-android/src/main/res/layout/list_item_image.xml