Context字面意思上下文，位于framework package的android.content.Context中，
其实该类为LONG型，类似Win32中的Handle句柄，很多方法需要通过Context才能识别调用者的实例。
官方说明：
Interface to global information about an application environment. This is an abstract class whose implementation is provided by the Android system. It allows access to application-specific resources and classes, as well as up-calls for application-level operations such as launching activities, broadcasting and receiving intents, etc.