hwc: Enable downscale feature through system property

Add support to turn the MDP downscale feature ON through the
use of a system property. 'sys.hwc.mdp_downscale_enabled' shall be
set to 'true' in build.prop file to enable this functionality.

This is required for OEMs that might have privacy concerns over panel
size information being exposed.

Change-Id: I354b25b1fdabe5786aed7552f2afccda35412467
diff --git a/libvirtual/virtual.cpp b/libvirtual/virtual.cpp
index 795d8a1..7d8b942 100644
--- a/libvirtual/virtual.cpp
+++ b/libvirtual/virtual.cpp
@@ -167,7 +167,13 @@
 
         initResolution(extW, extH);
 
-        if(!qdutils::MDPVersion::getInstance().is8x26()) {
+        // Dynamic Resolution Change depends on MDP downscaling.
+        // MDP downscale property will be ignored to exercise DRC use case.
+        // If DRC is in progress, ext WxH will have non-zero values.
+        bool isDRC = (extW > 0) && (extH > 0);
+
+        if(!qdutils::MDPVersion::getInstance().is8x26()
+                && (mHwcContext->mMDPDownscaleEnabled || isDRC)) {
 
             // maxArea represents the maximum resolution between
             // primary and virtual display.