[Bluetooth] Convert BluetoothController logs to log buffer so we'll

always have them.

We're mainly interested in the activeDeviceChanged log for b/243755182
but it'll be good to have all the logs.

This is almost the same as Id1b12dbf802cae07120f1d6cc2c0b2e3967a1c75,
but with these changes:
 - In BluetoothControllerImpl#onActiveDeviceChanged, check for null
 - Add @NonNull or @Nullable annotations to BluetoothCallback
 - Add a test case for #onActiveDeviceChanged with a null device

Bug: 246876230
Bug: 243755182
Fixes: 234774643
Test: dumped log buffer from device after connecting + disconnecting
bluetooth devices
Test: BluetoothControllerImplTest

Change-Id: I8643bd0061559c3cfc90f0741de12949be3f68f2