Fix Back Key behaviour and Director::getInstance()->end() on WP8
set default priority to avoid warning

when call 'scheduleUpdate' and 'unscheuleUpdate' several times in one frame, it will get warning "warning: you CANNOT change update priority in scheduled function"
Merge pull request #9158 from zhongfq/v3

set default priority to avoid warning
Merge pull request #9133 from pyrosphere/fix-wp8-back-key

Fix Back Key behaviour and Director::getInstance()->end() on WP8
[ci skip] Update CHANGELOG
Merge branch 'cocos_v3' into v3