Oculus 6dof manifest (#1310)

* Add separate Oculus 6DOF build target

* Add separate Add separate Oculus 6DOF build target, also in release mode.

Author: philip-lamb

app/src/oculusvrArmRelease/AndroidManifest.xml

@@ -4,7 +4,7 @@
     package="org.mozilla.vrbrowser"
     android:installLocation="auto">
     <uses-feature android:glEsVersion="0x00030001"/>
-    <uses-feature android:name="android.hardware.vr.headtracking" android:version="1" android:required="true" />
+    <uses-feature android:name="android.hardware.vr.headtracking" android:version="1" android:required="${headtrackingRequired}" />
     <application
         android:name=".VRBrowserApplication"
         android:allowBackup="true"