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/internal/window/handle.go

@@ -0,0 +1,7 @@
+// +build !android
+
+package window
+
+var PlatformHandle *Handle
+
+type Handle struct{}