Add sensor battery level warnings and readouts

[gordonel]

Is your feature request related to a problem? Please describe.
I'd like to be notified if it's time to charge my devices or replace the batteries in advance.

Describe the solution you'd like
When starting an activity, OpenTracks should read the battery level if available and notify the user if it falls below a user-specified level. Also, the app can provide convenient readouts in the notifications drawer as extra info

Describe alternatives you've considered
reading battery levels through nRF connect before a workout

[dennisguse]

Duplicate of #24, but way more detailed.

It is actually not that complicated anymore as passing data to the UI while recording became way easier.

[gordonel]

Eeeh, sorry about that. Do you think it would be best to close one of the dups in favour of the other?

[dennisguse]

@gordonel I really like your feature description; could you just add it to #24 and close this one?

[gordonel]

Done