Create ImageCompressor amd ImageCompressorImpl

* Methods from AttachmentCreationTask have been moved into them:
  * compressImage()
  * createBitmap()
* ImageCompressor is availabe via AttachmentModule

briar-api/src/main/java/org/briarproject/briar/api/test/TestAvatarCreator.java

@@ -1,10 +1,11 @@
 package org.briarproject.briar.api.test;
 
+import java.io.IOException;
 import java.io.InputStream;
 
 import javax.annotation.Nullable;
 
 public interface TestAvatarCreator {
 	@Nullable
-	InputStream getAvatarInputStream();
+	InputStream getAvatarInputStream() throws IOException;
 }