use imgdiff -z to diff zipfiles

imgdiff now understands the zip file format and can produce smaller
patches by doing the patching in uncompressed space.  Start taking
advantage of this for .zip, .apk, and .jar files.

tools/releasetools/ota_from_target_files

@@ -481,6 +481,15 @@ def GetRecoveryAPIVersion(zip):
     except KeyError:
       return 0
 
+
+DIFF_METHOD_BY_EXT = {
+    ".gz" : "imgdiff",
+    ".zip" : ["imgdiff", "-z"],
+    ".jar" : ["imgdiff", "-z"],
+    ".apk" : ["imgdiff", "-z"],
+    }
+
+
 def WriteIncrementalOTAPackage(target_zip, source_zip, output_zip):
   source_version = GetRecoveryAPIVersion(source_zip)
 
@@ -526,9 +535,8 @@ def WriteIncrementalOTAPackage(target_zip, source_zip, output_zip):
       verbatim_targets.append((fn, tf.size))
     elif tf.sha1 != sf.sha1:
       # File is different; consider sending as a patch
-      diff_method = "bsdiff"
-      if tf.name.endswith(".gz"):
-        diff_method = "imgdiff"
+      ext = os.path.splitext(tf.name)[1]
+      diff_method = DIFF_METHOD_BY_EXT.get(ext, "bsdiff")
       d = Difference(tf, sf, diff_method)
       if d is not None:
         print fn, tf.size, len(d), (float(len(d)) / tf.size)