app: add RegisterFragment method on *Window for Android

RegisterFragment creates an instance of a Java class and registers
it as a Fragment in the window's Context.

Signed-off-by: Greg Pomerantz <gmp.gio@wow.st>

app/app_android.go

@@ -0,0 +1,30 @@
+package app
+
+import (
+	"gioui.org/app/internal/window"
+)
+
+type Handle window.Handle
+
+// PlatformHandle returns the Android platform-specific Handle.
+func PlatformHandle() *Handle {
+	return (*Handle)(window.PlatformHandle)
+}
+
+// androidDriver is an interface that allows the Window's run method
+// to call the RegisterFragment method of the Android window driver.
+type androidDriver interface {
+	RegisterFragment(string)
+}
+
+// RegisterFragment constructs a Java instance of the specified class
+// and registers it as a Fragment in the Context in which the View was
+// created.
+func (w *Window) RegisterFragment(del string) {
+	go func() {
+		w.driverFuncs <- func() {
+			d := w.driver.(androidDriver)
+			d.RegisterFragment(del)
+		}
+	}()
+}