[AChoreographer] Add const qualifiers for getFd().

Per API feedback the AChoreographer argument should be const.

Bug: 148954590
Test: builds
Change-Id: Ie95481992534b3f5f5c664709bff80b1c21f7d50
diff --git a/libs/gui/DisplayEventDispatcher.cpp b/libs/gui/DisplayEventDispatcher.cpp
index 8af1a1c..15f966d 100644
--- a/libs/gui/DisplayEventDispatcher.cpp
+++ b/libs/gui/DisplayEventDispatcher.cpp
@@ -104,7 +104,7 @@
     mConfigChangeFlag = configChangeFlag;
 }
 
-int DisplayEventDispatcher::getFd() {
+int DisplayEventDispatcher::getFd() const {
     return mReceiver.getFd();
 }