libgui/SF: Propagate SECURE Layer flag changes

This allows changes to the SECURE flag to propagate down to
Layers in SurfaceFlinger so that WindowManager can change it on the fly
in response to device policy updates.

Bug: 20934462
Change-Id: I558f6d22c6273be373f1f480365e42536af18a33
diff --git a/services/surfaceflinger/Layer.h b/services/surfaceflinger/Layer.h
index 4e222b7..c1e5e9f 100644
--- a/services/surfaceflinger/Layer.h
+++ b/services/surfaceflinger/Layer.h
@@ -174,7 +174,7 @@
      * isSecure - true if this surface is secure, that is if it prevents
      * screenshots or VNC servers.
      */
-    virtual bool isSecure() const           { return mSecure; }
+    virtual bool isSecure() const;
 
     /*
      * isProtected - true if the layer may contain protected content in the
@@ -402,7 +402,6 @@
     mutable Texture mTexture;
 
     // page-flip thread (currently main thread)
-    bool mSecure; // no screenshots
     bool mProtectedByApp; // application requires protected path to external sink
 
     // protected by mLock