[android] Create attachments before showing previews

...a/org/briarproject/briar/android/blog/ReblogFragment.java

...riarproject/briar/android/blog/WriteBlogPostActivity.java

...ject/briar/android/conversation/AttachmentController.java

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

...ect/briar/android/conversation/ConversationViewModel.java

...iar/android/introduction/IntroductionMessageFragment.java

...iarproject/briar/android/sharing/BaseMessageFragment.java

...iarproject/briar/android/threaded/ThreadListActivity.java

...ava/org/briarproject/briar/android/view/ImagePreview.java

.../briarproject/briar/android/view/ImagePreviewAdapter.java

...org/briarproject/briar/android/view/ImagePreviewItem.java

...iarproject/briar/android/view/ImagePreviewViewHolder.java

...rproject/briar/android/view/TextAttachmentController.java

...g/briarproject/briar/android/view/TextSendController.java

briar-android/src/main/res/values/strings.xml

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

.../briarproject/briar/api/messaging/MessagingConstants.java

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

...rg/briarproject/briar/messaging/MessagingManagerImpl.java