Improve Dual-SIM support

Dual SIM phones are nice, but while they work more or less in SFOS, there’s room for improvement in some areas:

  1. Bluetooth HFP etc: A more or less random SIM is selected for outgoing calls (apparently it’s the “last active” SIM). This should be selectable in Bluetooth Settings. A global option would be sufficient, an additional per-pairing setting would be super nice. Asking per-connection is no-go (think cars).
  2. Seeing which SIM is which: Currently SIM card are hard coded as SIM1 and SIM2 in the UI. They should have user-friendly (user-set) names so it’s possible to see which does which.
  3. Documentation and instrumentation for developers to use the dual SIMs should be improved.