Refactor attachment loading to support incremental display once loaded

...ndroid/attachment/AttachmentRetrieverIntegrationTest.java

...oject/briar/android/attachment/AttachmentCreatorImpl.java

...briarproject/briar/android/attachment/AttachmentItem.java

...project/briar/android/attachment/AttachmentRetriever.java

...ect/briar/android/attachment/AttachmentRetrieverImpl.java

...riarproject/briar/android/attachment/UnavailableItem.java

...ject/briar/android/conversation/ConversationActivity.java

...t/briar/android/conversation/ConversationMessageItem.java

...riarproject/briar/android/conversation/ImageActivity.java

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

...riarproject/briar/android/conversation/ImageFragment.java

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

briar-android/src/main/res/drawable/ic_image_broken.xml

briar-android/src/main/res/drawable/ic_image_missing.xml

...ect/briar/android/attachment/AttachmentRetrieverTest.java

...rg/briarproject/briar/api/messaging/MessagingManager.java