Bump version to 4.75.4.

Accidentally went the wrong direction with canonicalVersionCode in
4.75.3. So this release just fixes that and uses the correct
canonicalVersionCode.

app/build.gradle

@@ -80,8 +80,8 @@ protobuf {
     }
 }
 
-def canonicalVersionCode = 726
-def canonicalVersionName = "4.75.3"
+def canonicalVersionCode = 728
+def canonicalVersionName = "4.75.4"
 
 def postFixSize = 10
 def abiPostFix = ['universal'   : 0,