Bug 40042: Add option do overwrite timestamp in extension version

app/build.gradle

@@ -679,7 +679,10 @@ ext.updateExtensionVersion = { task, extDir ->
         rename { 'manifest.json' }
         into extDir
 
-        def values = ['version': AndroidComponents.VERSION + "." + new Date().format('MMddHHmmss')]
+        def systemEnvBuildDate = System.getenv('MOZ_BUILD_DATE')
+        def values = ['version': AndroidComponents.VERSION + "." +
+                      (systemEnvBuildDate != null ? systemEnvBuildDate :
+                      new Date().format('MMddHHmmss'))]
         inputs.properties(values)
         expand(values)
     }