Refactor attachment creation to use injection.

...ndroid/attachment/AttachmentRetrieverIntegrationTest.java

...java/org/briarproject/briar/android/AndroidComponent.java

...arproject/briar/android/attachment/AttachmentCreator.java

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

...roject/briar/android/attachment/AttachmentDimensions.java

...arproject/briar/android/attachment/AttachmentManager.java

...iarproject/briar/android/attachment/AttachmentModule.java

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

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

...rg/briarproject/briar/android/attachment/ImageHelper.java

...riarproject/briar/android/attachment/ImageHelperImpl.java

...project/briar/android/attachment/ImageSizeCalculator.java

.../java/org/briarproject/briar/android/attachment/Size.java

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

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