error out when SurfaceTexture APIs are called while not connected

- also log a warning when freeAllBuffers is called with a non empty buffer queue
- rename freeAllBuffers to freeAllBuffersLocked

Change-Id: Idb71fdcf233b9ccae62d5a2a7c3c4bad2501d877
diff --git a/include/gui/SurfaceTexture.h b/include/gui/SurfaceTexture.h
index 3f9e68d..f93930d 100644
--- a/include/gui/SurfaceTexture.h
+++ b/include/gui/SurfaceTexture.h
@@ -210,7 +210,7 @@
 
     // freeAllBuffers frees the resources (both GraphicBuffer and EGLImage) for
     // all slots.
-    void freeAllBuffers();
+    void freeAllBuffersLocked();
     static bool isExternalFormat(uint32_t format);
 
 private: