fix(prepare): delete splash screens if none are used (apache#1227)

Currently, we copy default splash screens to the platform folder during platform creation, but then we do not delete them when the user uses no splash screens at all. This causes apache#1226 and what is described in apache#689.

The new implementation does not return early when updating splashes and none are defined. Instead, we let the cleanup map we create initially take care of deleting all unused splash screens. This also deletes the default splashes during the first prepare after platform creation.

Fixes apache#1226

bin/templates/cordova/lib/prepare.js

@@ -323,10 +323,13 @@ function makeSplashCleanupMap (projectRoot, resourcesDir) {
 function updateSplashes (cordovaProject, platformResourcesDir) {
     var resources = cordovaProject.projectConfig.getSplashScreens('android');
 
-    // if there are "splash" elements in config.xml
+    // if there are no "splash" elements in config.xml
     if (resources.length === 0) {
         events.emit('verbose', 'This app does not have splash screens defined');
-        return;
+        // We must not return here!
+        // If the user defines no splash screens, the cleanup map will cause any
+        // existing splash screen images (e.g. the defaults that we copy into a
+        // new app) to be removed from the app folder, which is what we want.
     }
 
     // Build an initial resource map that deletes all existing splash screens