SF: Split Scheduler::setRefreshRateSelector

Add helper functions to bind/unbind the idle timer.

Bug: 255635821
Test: Build (-Wthread-safety)
Change-Id: I68cd1274e2b0591652a259b7f60d0a370883e512
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index 5306602..864245a 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -2941,6 +2941,8 @@
 
         // Display modes are reloaded on hotplug reconnect.
         if (display->isPrimary()) {
+            // TODO(b/241285876): Annotate `processDisplayAdded` instead.
+            ftl::FakeGuard guard(kMainThreadContext);
             mScheduler->setRefreshRateSelector(selectorPtr);
         }