SF: Prevent crashes in onRefreshRateChangedDebug

The AIDL thread reads HWComposer state without locks, but there may be
a concurrent write on the main thread. Also, the display may no longer
exist by the time the scheduled task runs.

Bug: 241285876
Test: presubmit
Change-Id: I3460e23cad2a884250bebcd58b1eaec9cd309858