Remove CloseGuard from DisplayEventReceiver

Majority usage of DisplayEventReceiver is with Choreographer, which
is a thread-local. As such, it cannot have dispose() called on it
easily and dispose is also @hide. Since apps can't do anything
about this, and aren't really supposed to, remove the CloseGuard
as it just generates false-positive errors for 3p devs.

Fixes: 154358085
Test: none
Change-Id: I5e9e5324644967897f2b37a19f4fb1485b8ae5bb