Fix one NPE in tethering setting

Once we not declare BT feature, go Setting->more->tethering
will trigger this NPE.
Cherry-picked from AOSP: https://android-review.googlesource.com/#/c/63695/

Change-Id: I4736e7bade261475acda74e26250a81452018ffa
Signed-off-by: Jianzheng Zhou <jianzheng.zhou@freescale.com>