Change Android target platform of Cocos2d-x 3.x projects

Manually change the target platform ID in:
[Project Root]/cocos2d/cocos/platform/android/java/project.properties
[Project Root]/proj.android/project.properties

Then compile it again.
cocos compile -p android --ap [Platform ID]

Cocos2d-x 3.x still haven't provide a clean directive, so you might need to clean compiled obj manually.