[bluetooth] Fix disappearing bluetooth devices in Inbox

[PRosenb]

I had troubles with detecting Bluetooth devices. On first scan, it listed multiple devices but when I did the scan again a bit later, the list was mostly empty.

The code that handles Bluetooth device detection is quite complex (too complex I feel). I analysed the code and used the debugger to get a basic understanding.

Suggested changes to improve Bluetooth device detection:

• Notify Things again when they are removed from listeners like Inbox

[cpmeister]

The rssi is highly variable and can change many times a second. Discovery recalculation should only occur if the previous result expired or there is new immutable information about the device that might cause it to get discovered differently. Signal strength is not "immutable" information and as such should not cause a new discovery attempt.

[PRosenb]

Thanks for the explanation. Yes, that could change too often then.
I removed the RSSI related commit.

[kaikreuzer]

Thanks @PRosenb, this is awesome! I had exactly the same issue and tried to fix it with #10154, which miserably failed.
I tested your fix and can confirm that it is working nicely - I now have a stable list of devices in the inbox on subsequent scans.
@cpmeister Ieave merging to you as you're the expert for the code in question.
If you allow, I have to agree with @PRosenb that the discovery code is tough to read and it gave me a hard time - so hard that I didn't feel comfortable of doing changes to it...