Reuse BluetoothPan object and use it under tethering handler thread

1. Instead of create BluetoothPan every time when tethering need to use
it, store it with mBluetoothPan and resue it.
2. Call BluetoothPan function under tethering handler thread.

Bug: 190438212
Test: atest TetheringTests

Change-Id: I40adece59960ec44a02dc438d6bd95483a0788af
2 files changed