Add null check when receiving PAN connection state changed

In some corner cases, when the Bluetooth is turning off, getDevice will
return null since mAdapter is null. Add null check when receiving PAN
connection state changed.

Bug: 150522777
Test: atest BluetoothInstrumentationTests
Tag: #stability

Change-Id: Ifc0ca5d505febaae7e1bfae63dc69743fc4ece37