Ensure onHandleDestroyed drops reference before releasing lock.

onHandleDestroyed copies the layer pointer and references it in
mLayersPendingRemoval, so that the final reference will be dropped
from the main thread. However the current implementation is flawed.
It is possible for the main thread to process mLayersPendingRemoval
after we release the lock, but BEFORE we finish executing the
~LayerCleaner destructor. This would mean the last reference would
again be LayerCleaner::mLayer and we would incorrectly run the d'tor
on the main thread. Scheduling priority boosts to the main thread
make this more likely than it may seem.

Test: Boots. Existing tests pass.
Change-Id: Ife247b530b67907b28b4f29f6766c2056fe49bfa
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index d3f0ece..b1bfb3a 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -601,7 +601,7 @@
     // called when all clients have released all their references to
     // this layer meaning it is entirely safe to destroy all
     // resources associated to this layer.
-    void onHandleDestroyed(const sp<Layer>& layer);
+    void onHandleDestroyed(sp<Layer>& layer);
 
     // remove a layer from SurfaceFlinger immediately
     status_t removeLayer(const sp<Layer>& layer);