Keep the display event receiver around forever.

There is really no point disposing the display event receiver
anymore.  Moreover, it's hard to choose a good time to do it
since the Choreographer only supports one-shot callbacks now.

So let's made the code simpler.

Bug: 5721047
Change-Id: I8533a54e93a787e0ca30d99a1f1eea85534b13b9