add support to specify activity, if social app offers multiple sharing ways

You can specify the activity if the app offers multiple sharing ways, just passing 'packageName/activityName' to shareVia. 

For example, WeChat, passing 'com.tencent.mm' or 'com.tencent.mm/com.tencent.mm.ui.tools.ShareImgUI' to share to chat, passing 'com.tencent.mm/com.tencent.mm.ui.tools.ShareToTimeLineUI' to share to moments.