gps on banglejs2: report satellites before the fix

### Affected hardware version

Bangle 2

### Your firmware version

2v19

### The bug

GPS fix (as reported by Bangle.on('GPS', function(fix) { ... });) contains satellites field. Unfortunately it is only nonzero when we actually have a fix.

To provide some kind of feedback before fix is acquired, it would be nice to know number of satellites decoded even before we have the fix. (Currently such information is only available with gps-raw interface). There's big difference between "no fix, 6 sats decoded" -- which means "stay in place and wait a minute" and "no fix, 0 sats" which means "move under clear sky, can't get a fix here".

My proposal would be to report fix.satellites as number of satellites with known snr ratio when we don't have a fix, but other solutions are possible.

### Installed apps

_No response_

Start by tracing the Bangle.on('GPS', function(fix) { ... }) interface and compare its satellites field with the gps-raw interface described in the issue. Determine how satellite counts are represented before a fix, then verify that the reported count distinguishes no fix with decoded satellites from no satellites.

Written by the indexing model from the issue text.