Fix DummyNativeWindow to respond to new query

The `NATIVE_WINDOW_IS_VALID` query must now return true in order for
`eglCreateWindowSurface` to succeed. Technically this isn't true for our
dummy window, but we only need it to create the context, it gets dropped
immediately afterwards.

Bug: 36102224
Test: Created a simple test application. Also ran a Vr application with
      these changes applied.

Change-Id: I2d559962d28db4b1cb6ad188269e4e54cf47245d