Revert "Bluetooth: Fix to update mTetherOn with proper value."

This reverts commit 3621c8d51d2f7ae4f0aff5d0b01d52fb4b685810.
Revert "Bluetooth: Added a property to have the tethering setting persist"

This reverts commit 977054484a7a71579d12a722f4feec68caf079fa.
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