The refactored DiscoveryActivityV2 does not freeze anymore, even under
maximum scanning intensity on older phones, and seems to not have
introduced any regressions.
- get rid of pre-lollipop BLE discovery (nowadays we support only lollipop and above) and related preference
- get rid of the sequenced BT-then-BLE-scan that wasn't working reliably anyway and was causing a recursion
- add a caching layer for already processed devices (the same device is found multiple times during discovery)
- add a caching layer for device name in GBDeviceCandidate (many coordinators will ask for it, and it's a very expensive operation)
