Bluetooth: Set tethering global state while starting the Pan service

When Remote device(PANU) try to reconnect DUT(NAP) after just
BT turned on(SSR case OR normal BT off/on case) without going to
tetheringSetting menu on DUT, So It's failed to connect. Even though
BT tethering was enabled before turning off BT. As tetheringSetting
is an activity, So it does not receive ACTION_STATE_CHANGED indent,
When it's out of focus(activity is not there), So once BT is turned
on again, isTetheringOn is not getting triggered from tethering
Setting activity(As activity is not there), which is responsible to
update tethering global state for PAN service after BT on. Tethering
global state(mTetherOn) must be set based on the last saved tethering
preference while starting the Pan service to solve this issue.

Change-Id: I51d36751c094b726fff7415bde6ea1b7af341c76
CRs-Fixed: 532893