surfaceflinger: protect Client::mParentLayer with a lock

Updates to wp<> is not atomic.  Use Client::mLock to protect
mParentLayer.

Bug: 38505866
Test: camera and youtube work
Change-Id: I2739382d5bb99961a47c1011963b6f676d34eec6
diff --git a/services/surfaceflinger/Client.h b/services/surfaceflinger/Client.h
index b5f98b8..2aab28f 100644
--- a/services/surfaceflinger/Client.h
+++ b/services/surfaceflinger/Client.h
@@ -71,12 +71,13 @@
     virtual status_t onTransact(
         uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags);
 
+    sp<Layer> getParentLayer(bool* outParentDied = nullptr) const;
+
     // constant
     sp<SurfaceFlinger> mFlinger;
 
     // protected by mLock
     DefaultKeyedVector< wp<IBinder>, wp<Layer> > mLayers;
-
     wp<Layer> mParentLayer;
 
     // thread-safe