Bug 40013: [android] Add option do overwrite timestamp in extension version

mobile/android/shared-settings.gradle

@@ -163,7 +163,15 @@ gradle.projectsLoaded { ->
                 rename { 'manifest.json' }
                 into extDir
 
-                def values = ['version': getManifestVersionString(rootProject.ext.config.componentsVersion)]
+                def systemEnvBuildDate = System.getenv('MOZ_BUILD_DATE')
+                
+                // MOZ_BUILD_DATE is in the YYYYMMDDHHMMSS format. Thus, we only use a
+                // substring of it if it is available.
+                def values = ['version': rootProject.ext.config.componentsVersion + "." +
+                        (systemEnvBuildDate != null ?
+                                systemEnvBuildDate.substring(4) :
+                                new Date().format('MMddHHmmss'))]
+
                 inputs.properties(values)
                 expand(values)
             }