Invoke a padding round in certain use-cases.

* In some use-cases, it is possible that there are
no AppBuffer layers on the external/virtual layer-list
during which all the pipes will be allocated to the
primary. When layers do comeup on external/virtual
layer-list, subsequent overlay sets fail.

* This change ensures that in such cases, we invoke a
padding round on all the displays to free up the
hw resources which can be used in subsequent cycles.

Change-Id: Ifac0b8f51a8719eb55b11010d05b8d11352db054

libhwcomposer/hwc_utils.h

@@ -483,6 +483,10 @@ struct hwc_context_t {
     eAnimationState mAnimationState[HWC_NUM_DISPLAY_TYPES];
     qhwc::HWCVirtualBase *mHWCVirtual;
 
+    // stores the #numHwLayers of the previous frame
+    // for each display device
+    int mPrevHwLayerCount[HWC_NUM_DISPLAY_TYPES];
+
     // stores the primary device orientation
     int deviceOrientation;
     //Securing in progress indicator