Android: Define a public API in Java

The Java code in Qt currently comes with no binary or source compatibility guarantees. To make it convenient to write platform-specific code with Qt, we need to define a public API with these guarantees.

This applies both to some of the code in the .jar file (like getting Qt's layout etc.) which can easily be placed in the library code, but also to the QtActivity class itself, where we need to add official places where you can hook into event handling and startup.