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_android.go

@@ -0,0 +1,11 @@
+package window
+
+var PlatformHandle *Handle
+
+type Handle struct {
+	// JVM is the JNI *JVM pointer.
+	JVM uintptr
+	// Context is a global reference to the application's
+	// android.content.Context instance.
+	Context uintptr
+}