If the PAN service disconnects before the BT iface is available, the BT
TetheringRequest will be stuck pending and prevent any new BT
startTethering attempts. This may happen if the user turns BT tethering
on and turns off BT without connecting to any device.
Fix this by always removing the pending BT request when the PAN service
disconnects.
Flag: EXEMPT minor bugfix
Bug: 420412916
Test: atest TetheringTest, manually verify turning BT tethering on,
turning off BT, and turning BT tethering on again.
Change-Id: I6bc96d6e4016d83af6cfd8e549d474ce651a44fc
7fe8e5653f