app: add Handle type and PlatformHandle()

On Android, allow access to JVM and Application context.

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

app/app.go

@@ -9,6 +9,13 @@ import (
 	"gioui.org/app/internal/window"
 )
 
+type Handle window.Handle
+
+// PlatformHandle returns the platform specific Handle.
+func PlatformHandle() *Handle {
+	return (*Handle)(window.PlatformHandle)
+}
+
 // extraArgs contains extra arguments to append to
 // os.Args. The arguments are separated with |.
 // Useful for running programs on mobiles where the