Platform: {Android,Glfw}Application: consistent windowSize() with SDL.

Not providing framebufferSize() or dpiScaling() there yet, as that's a
bit involved in both cases. Support will be added, eventually.

src/Magnum/Platform/AndroidApplication.cpp

@@ -165,7 +165,7 @@ bool AndroidApplication::tryCreate(const Configuration& configuration, const GLC
     return _context->tryCreate();
 }
 
-Vector2i AndroidApplication::windowSize() {
+Vector2i AndroidApplication::windowSize() const {
     return {ANativeWindow_getWidth(_state->window),
             ANativeWindow_getHeight(_state->window)};
 }

src/Magnum/Platform/AndroidApplication.h

@@ -300,13 +300,15 @@ class AndroidApplication {
 
         /** @{ @name Screen handling */
 
+    public:
         /**
          * @brief Window size
          *
          * Window size to which all input event coordinates can be related.
          */
-        Vector2i windowSize();
+        Vector2i windowSize() const;
 
+    protected:
         /**
          * @brief Swap buffers
          *

src/Magnum/Platform/GlfwApplication.cpp

@@ -424,7 +424,7 @@ GlfwApplication::~GlfwApplication() {
     glfwTerminate();
 }
 
-Vector2i GlfwApplication::windowSize() {
+Vector2i GlfwApplication::windowSize() const {
     Vector2i size;
     glfwGetWindowSize(_window, &size.x(), &size.y());
     return size;

src/Magnum/Platform/GlfwApplication.h

@@ -330,17 +330,17 @@ class GlfwApplication {
 
         /** @{ @name Screen handling */
 
-        #ifndef CORRADE_TARGET_EMSCRIPTEN
+    public:
         /**
          * @brief Window size
          *
          * Window size to which all input event coordinates can be related.
          * Note that especially on HiDPI systems the reported window size might
          * not be the same as framebuffer size.
          */
-        Vector2i windowSize();
-        #endif
+        Vector2i windowSize() const;
 
+    protected:
         /**
          * @brief Swap buffers
          *