Fixes #2022, do not resume on Android when surfaceChanged

If the app is in landscape mode and the user presses the power button, a pause
is followed immediately by a surfaceChanged event because the lock screen
is shown in portrait mode. This triggers a "false" resume.
So, we just pause and resume following the onWindowFocusChanged events.

Also, wait for SDL_APP_WILLENTERBACKGROUND and SDL_APP_DIDENTERBACKGROUND before
blocking the event pump.

test/testnativex11.c

@@ -0,0 +1,53 @@
+/*
+  Copyright (C) 1997-2013 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely.
+*/
+
+#include "testnative.h"
+
+#ifdef TEST_NATIVE_X11
+
+static void *CreateWindowX11(int w, int h);
+static void DestroyWindowX11(void *window);
+
+NativeWindowFactory X11WindowFactory = {
+    "x11",
+    CreateWindowX11,
+    DestroyWindowX11
+};
+
+static Display *dpy;
+
+static void *
+CreateWindowX11(int w, int h)
+{
+    Window window = 0;
+
+    dpy = XOpenDisplay(NULL);
+    if (dpy) {
+        window =
+            XCreateSimpleWindow(dpy, DefaultRootWindow(dpy), 0, 0, w, h, 0, 0,
+                                0);
+        XMapRaised(dpy, window);
+        XSync(dpy, False);
+    }
+    return (void *) window;
+}
+
+static void
+DestroyWindowX11(void *window)
+{
+    if (dpy) {
+        XDestroyWindow(dpy, (Window) window);
+        XCloseDisplay(dpy);
+    }
+}
+
+#endif