Merge "Prevents exceptions while putting stacks to sleep"

        for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {

            final ActivityDisplay display = mActivityDisplays.get(displayNdx);

            for (int stackNdx = display.getChildCount() - 1; stackNdx >= 0; --stackNdx) {

                // Stacks and activities could be removed while putting activities to sleep if

                // the app process was gone. This prevents us getting exception by accessing an

                // invalid stack index.

                if (stackNdx >= display.getChildCount()) {

                    continue;

                }

                final ActivityStack stack = display.getChildAt(stackNdx);

                if (allowDelay) {

                    allSleep &= stack.goToSleepIfPossible(shuttingDown);