Add multiple windows support for Android

[Hagb]

With toga part: https://github.com/beeware/toga/pull/1394

What have been done:

• Codes in MainActivity about a window are moved to WinActivity, and MainActivity becomes the subclass of WinActivity;
• A listener object (IPythonWin) is now for a window but not for the whole application;
• Use intent to create new window, with the python id of the listener object (IPythonWin) passed, and then the id will be used to get (IPythonApp.getPythonWinById) the listener of the new window;
• Add onDestory event.

PR Checklist:

• [ ] All new features have been tested
• [ ] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct

[freakboy3742]

Closing as the Toga ticket related to this has also been closed.