PluginsRegister 不再对外暴露，需要手动注册插件请使用  FlutterBoost.instance().engineProvider().getPlugins().add(new FlutterPlugin());

修正present问题并添加测试用例

2. fix present页面僵死问题
3. 增加present的测试用例

[Android] 在createEngine增加一个回调，方便在使用测做一些事情(打点之类)

[bugfix] fix crash：'FlutterBoostPlugin not register yet'

         // case: initFlutter after Activity.OnCreate method，and then called start/stop crash
         // In SplashActivity ,showDialog(in OnCreate method) to check permission, if authorized, then init sdk and jump homePage)
         // with this case, do initFlutter on SecondActivity.OnCreate

Update README.md / README_CN.md

Update Boost version desc.

[sync] 2020/1/19

# Conflicts:
#	ios/Classes/Boost/FlutterBoostPlugin.h
#	lib/flutter_boost.dart

新的注册方式在onAttach时执行attachToView

Merge branch 'feature/base_flutter_1.12_upgrade' of github.com:gws321/flutter_boost into feature/base_flutter_1.12_upgrade