when disconnectHeadsetInternal, allow it when the BT is not disabled

The old code allowed the disconnection only when BT is enabled. This
caused a bug during BT turning off that Headset connection failed
to disconnect.
Change-Id: Iba15a1595b6987064f071ad90bbb126a6edfe6b5
diff --git a/framework/java/android/bluetooth/BluetoothHeadset.java b/framework/java/android/bluetooth/BluetoothHeadset.java
index 3284361..8f2b3d8 100644
--- a/framework/java/android/bluetooth/BluetoothHeadset.java
+++ b/framework/java/android/bluetooth/BluetoothHeadset.java
@@ -661,7 +661,7 @@
      */
     public boolean disconnectHeadsetInternal(BluetoothDevice device) {
         if (DBG) log("disconnectHeadsetInternal");
-        if (mService != null && isEnabled()) {
+        if (mService != null && !isDisabled()) {
             try {
                  return mService.disconnectHeadsetInternal(device);
             } catch (RemoteException e) {Log.e(TAG, e.toString());}