Properly register the input devices on Android

In https://code.qt.io/cgit/qt/qtbase.git/tree/src/plugins/platforms/android/androidjniinput.cpp#n299 getTouchDevice() calls registerInputDevice(), and getTouchDevice() is called only from touchEnd() and touchCancel(). But all platforms should register their input devices as soon as they are discovered (preferably at startup), and every input event should have its QInputEvent::device() being the actual device that the event comes from. This is what we depend on to recognize that a synthesized mouse event actually comes from a touchscreen, or a stylus, for example: it's a QMouseEvent but device() says it's really from a different device, not a mouse.