SkBitmapHasher: use 64-bit-truncated MD5 instead of 64-bit CityHash

gm/tests/outputs/compared-against-identical-pixels-images/output-expected/json-summary.txt

Index: gm/tests/outputs/compared-against-identical-pixels-images/output-expected/json-summary.txt
===================================================================
--- gm/tests/outputs/compared-against-identical-pixels-images/output-expected/json-summary.txt	(revision 8985)
+++ gm/tests/outputs/compared-against-identical-pixels-images/output-expected/json-summary.txt	(working copy)
@@ -5,20 +5,20 @@
       "no-comparison" : null,
       "succeeded" : {
          "565/selftest1" : {
-            "bitmap-cityhash" : 9512553915271796906
+            "bitmap-cityhash" : 12927999507540085554
          },
          "8888/selftest1" : {
-            "bitmap-cityhash" : 14022967492765711532
+            "bitmap-cityhash" : 1209453360120438698
          }
       }
    },
    "expected-results" : {
       "565/selftest1" : {
-         "allowed-bitmap-cityhashes" : [ 9512553915271796906 ],
+         "allowed-bitmap-cityhashes" : [ 12927999507540085554 ],
          "ignore-failure" : false
       },
       "8888/selftest1" : {
-         "allowed-bitmap-cityhashes" : [ 14022967492765711532 ],
+         "allowed-bitmap-cityhashes" : [ 1209453360120438698 ],
          "ignore-failure" : false
       }
    }