Android: request to show SW keyboard not cleared up when view destroyed

Running the test case tst_QGraphicsProxyWidget::focusProxy_QTBUG_51856() on Android leads to many calls to show the software keyboard, which in Android QPA plugin is done by posting a Runnable to handle it on the Android main event queue, with a 15 ms delay. Since this is done on the Activity level, when the view and window leading to the call are destroyed, those Runnables are not cleared up from the event queue, leading to the keyboard being shown during subsequent, unrelated test cases, causing random failures.

Those calls should be cleared away when the view/window they originate from is destroyed. Could also be worthwhile to not make the call a new every time.