Factor out the image classes to break circular dependency

This helps to break the circular dependency between common and
blockimgdiff.

Bug: 32379627
Test: unit tests pass
Change-Id: I90b5ff34782acbfac86f36265bd96c207d898bf6

tools/releasetools/test_blockimgdiff.py

@@ -18,9 +18,8 @@ import os
 from hashlib import sha1
 
 import common
-from blockimgdiff import (
-    BlockImageDiff, DataImage, EmptyImage, FileImage, HeapItem, ImgdiffStats,
-    Transfer)
+from blockimgdiff import BlockImageDiff, HeapItem, ImgdiffStats, Transfer
+from images import DataImage, EmptyImage, FileImage
 from rangelib import RangeSet
 from test_utils import ReleaseToolsTestCase