Include IMGDIFF operation in minor version 4 or up.

Try imgdiff if both source and target contains gzip.

Test: unit test & generated a payload with boot.img
Bug: 26675118
Change-Id: I2861d9b953ffbdec44fdfb42cff5687698ea205e
diff --git a/payload_consumer/payload_constants.cc b/payload_consumer/payload_constants.cc
index e47b4f8..3d78e0a 100644
--- a/payload_consumer/payload_constants.cc
+++ b/payload_consumer/payload_constants.cc
@@ -25,6 +25,7 @@
 const uint32_t kInPlaceMinorPayloadVersion = 1;
 const uint32_t kSourceMinorPayloadVersion = 2;
 const uint32_t kOpSrcHashMinorPayloadVersion = 3;
+const uint32_t kImgdiffMinorPayloadVersion = 4;
 
 const char kLegacyPartitionNameKernel[] = "boot";
 const char kLegacyPartitionNameRoot[] = "system";