Add feature tag android.hardware.vr.headtracking.

Different versions of this feature specify different levels of
headtracking capability.

Bug: 36294374
Test: build
Change-Id: I3ca56822c9f8a0d48431762b7c4eba5cdf904512
diff --git a/data/etc/handheld_core_hardware.xml b/data/etc/handheld_core_hardware.xml
index 467e0fd..ed3db5c 100644
--- a/data/etc/handheld_core_hardware.xml
+++ b/data/etc/handheld_core_hardware.xml
@@ -56,6 +56,9 @@
     <!-- Devices with all optimizations required to be a "VR Ready" device that
          pass all CTS tests for this feature must include feature
          android.hardware.vr.high_performance -->
+    <!-- Devices that support VR headtracking features and pass all CDD
+         requirements may include
+         android.hardware.vr.headtracking -->
 
     <!-- devices with GPS must include android.hardware.location.gps.xml -->
     <!-- devices with an autofocus camera and/or flash must include either