Bluetooth: Added Shared Preferences to restore tethering setting

Added a Shared Preferences to have the tethering setting persist,
so that during the BT on/off the value can be restored and based
on the value tethering settings will be restored/altered.

CRs-fixed: 506194
Change-Id: I3fffbb78712f4177178531c6c202b786bfcd9d39