ui/app,apps/gophers,apps/hello: revert NewWindow to CreateWindow

It turns out we already support multiple windows on Android: when
the activity is recreated.

This reverts commit f21b5eb1df.

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

ui/app/os_android.go

@@ -55,8 +55,6 @@ 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 {
@@ -381,6 +379,6 @@ func Main() {
 	panic("unreachable")
 }
 
-func createWindow(opts *WindowOptions) (*Window, error) {
-	return <-windows, nil
+func createWindow(opts *WindowOptions) error {
+	return errors.New("createWindow not supported")
 }