[Android][SysApps] Change event names according recent spec changes.

Refer to https://github.com/sysapps/device-capabilities/issues/14, the event
names changed as below:
    attribute EventHandler  onstorageattach;     // was onattach
    attribute EventHandler  onstoragedetach;     // was ondetach
    attribute EventHandler  ondisplayconnect;    // was onconnect
    attribute EventHandler  ondisplaydisconnect; // was ondisconnect

At the same time, fixed a bug to allow pass null as callback of "onEventName".
Merge pull request #1277 from itlodge/fix_listener_logic

[Android][SysApps] Change event names according recent spec changes.