Add feature tag android.hardware.vr.headtracking.

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

Bug: 36294374
Test: build
Change-Id: I00273c9093321661b5ca519d7cd065ae0105c3b2

core/java/android/content/pm/PackageManager.java

@@ -2354,6 +2354,13 @@ public abstract class PackageManager {
     public static final String FEATURE_VR_MODE_HIGH_PERFORMANCE
             = "android.hardware.vr.high_performance";
 
+    /**
+     * Feature for {@link #getSystemAvailableFeatures} and {@link #hasSystemFeature}:
+     * The device implements headtracking suitable for a VR device.
+     */
+    @SdkConstant(SdkConstantType.FEATURE)
+    public static final String FEATURE_VR_HEADTRACKING = "android.hardware.vr.headtracking";
+
     /**
      * Action to external storage service to clean out removed apps.
      * @hide