Fix race condition clearing VSYNC enable on VrFlinger startup.

Fix VrFlinger to avoid touching the VSYNC enable setting in HWC
during startup by avoiding calling the cleanup code when entering
idle state for the first time.

Bug: 63626797
Test: Manual testing.
Change-Id: I5320bd5c1febb51367e759a8619d35f155789628
diff --git a/libs/vr/libvrflinger/hardware_composer.cpp b/libs/vr/libvrflinger/hardware_composer.cpp
index d937c88..def9b7d 100644
--- a/libs/vr/libvrflinger/hardware_composer.cpp
+++ b/libs/vr/libvrflinger/hardware_composer.cpp
@@ -821,8 +821,9 @@
       std::unique_lock<std::mutex> lock(post_thread_mutex_);
       ALOGI("HardwareComposer::PostThread: Entering quiescent state.");
 
-      // Tear down resources.
-      OnPostThreadPaused();
+      // Tear down resources if necessary.
+      if (was_running)
+        OnPostThreadPaused();
 
       was_running = false;
       post_thread_resumed_ = false;