Improve BT slice card loading performance

The bottleneck is getting LocalBluetoothManager the first time.
1. Initialize LocalBluetoothManager earlier and asynchronously.
2. Don't block in slice's constructor and getSlice().
  - Initialize the bt updaters until the manager is ready.
  - Just show a header if the manager is not ready yet.

Fixes: 157702021
Test: robotest
Change-Id: I427df55f259b45ba4c37557b22e09dcc24079e93