screenshots could stop working after camera was used

once a secure window is put on screen the display
would retain its "secure" flag forever, preventing
screenshots from being taken.

Bug: 6933967
Change-Id: I5be8355145ca7d580d84552311642f8fa912fe6a
diff --git a/services/surfaceflinger/DisplayDevice.cpp b/services/surfaceflinger/DisplayDevice.cpp
index e3655f8..cdc8a9f 100644
--- a/services/surfaceflinger/DisplayDevice.cpp
+++ b/services/surfaceflinger/DisplayDevice.cpp
@@ -244,6 +244,7 @@
 
 void DisplayDevice::setVisibleLayersSortedByZ(const Vector< sp<LayerBase> >& layers) {
     mVisibleLayersSortedByZ = layers;
+    mSecureLayerVisible = false;
     size_t count = layers.size();
     for (size_t i=0 ; i<count ; i++) {
         if (layers[i]->isSecure()) {