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
diff --git a/tools/releasetools/test_common.py b/tools/releasetools/test_common.py
index 50fa86f..c0ebd89 100644
--- a/tools/releasetools/test_common.py
+++ b/tools/releasetools/test_common.py
@@ -25,9 +25,9 @@
 import common
 import test_utils
 import validate_target_files
+from images import EmptyImage, DataImage
 from rangelib import RangeSet
 
-from blockimgdiff import EmptyImage, DataImage
 
 KiB = 1024
 MiB = 1024 * KiB