libgui: New mutex for ConsumerBase frame callbacks

Adds a new mutex to protect ConsumerBase's mFrameAvailableListener
separately from the main mMutex. This prevents deadlocks when trying
to perform callbacks in response to other calls that come through
ConsumerBase.

Bug: 62915791
Test: EncodeVirtualDisplayWithCompositionTest#
          testRenderingMaxResolutionRemotely

Change-Id: I244dc2d83488e452d9690c0d7847c9ff15a95dbb
diff --git a/services/surfaceflinger/SurfaceFlingerConsumer.cpp b/services/surfaceflinger/SurfaceFlingerConsumer.cpp
index 9babeef..abc8fde 100644
--- a/services/surfaceflinger/SurfaceFlingerConsumer.cpp
+++ b/services/surfaceflinger/SurfaceFlingerConsumer.cpp
@@ -240,10 +240,15 @@
 }
 
 void SurfaceFlingerConsumer::onSidebandStreamChanged() {
+    FrameAvailableListener* unsafeFrameAvailableListener = nullptr;
+    {
+        Mutex::Autolock lock(mFrameAvailableMutex);
+        unsafeFrameAvailableListener = mFrameAvailableListener.unsafe_get();
+    }
     sp<ContentsChangedListener> listener;
     {   // scope for the lock
         Mutex::Autolock lock(mMutex);
-        ALOG_ASSERT(mFrameAvailableListener.unsafe_get() == mContentsChangedListener.unsafe_get());
+        ALOG_ASSERT(unsafeFrameAvailableListener == mContentsChangedListener.unsafe_get());
         listener = mContentsChangedListener.promote();
     }