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/Android.bp b/tools/releasetools/Android.bp
index 6b4e4f5..1bb1603 100644
--- a/tools/releasetools/Android.bp
+++ b/tools/releasetools/Android.bp
@@ -40,6 +40,7 @@
         "common.py",
         "edify_generator.py",
         "img_from_target_files.py",
+        "images.py",
         "make_recovery_patch.py",
         "merge_target_files.py",
         "ota_from_target_files.py",