SurfaceFlinger: add explicit register for DISPLAY_EVENT_CONFIG_CHANGED

When display refresh rate changes, SF fires DISPLAY_EVENT_CONFIG_CHANGED
thru DisplayEventReceiver. If the other end of the pipe doesn't consume
the events it may lead to pipe full and dropping of events. Furthermore,
The only clients interested in this event in DisplayManager and hwui.

To avoid spamming all clients with this event, this change is adding an
explicit register for DISPLAY_EVENT_CONFIG_CHANGED events.

Bug: 131688378
Test: adb shell /data/nativetest64/libsurfaceflinger_unittest/libsurfaceflinger_unittest
Test: trigger config change and observe logcat
Change-Id: I5973a1ecc1f3e3ff8d8a0cba19db0e49ef0d5341