ui/app,apps/gophers,apps/hello: replace CreateWindow with NewWindow

Gio doesn't support multiple windows anyway, so get rid of the
app.Windows channel and use NewWindow call for the mobile platforms
as well.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/os_android.go

@@ -55,6 +55,8 @@ type window struct {
 
 var theJVM *C.JavaVM
 
+var windows = make(chan *Window)
+
 var views = make(map[C.jlong]*window)
 
 func jniGetMethodID(env *C.JNIEnv, class C.jclass, method, sig string) C.jmethodID {
@@ -379,6 +381,6 @@ func Main() {
 	panic("unreachable")
 }
 
-func createWindow(opts WindowOptions) error {
-	return errors.New("createWindow not supported")
+func createWindow(opts WindowOptions) (*Window, error) {
+	return <- windows, nil
 }