Allow plugins to set ext.multiarch to enable multiple APK building

The xwalk webView need build multiple apks by default after install
cordova-crosswalk-engine plugin, we can set ext.multiarch=true to open
the flag in plugin, it don't necessary set system environment BUILD_MULTIPLE_APKS
manually.

This closes #141

bin/templates/project/build.gradle

@@ -54,11 +54,11 @@ task wrapper(type: Wrapper) {
     gradleVersion = '2.2.1'
 }
 
+ext.multiarch=false
+
 // PLUGIN GRADLE EXTENSIONS START
 // PLUGIN GRADLE EXTENSIONS END
 
-ext.multiarch=false
-
 android {
     sourceSets {
         main {