Fallback to default SIM instead of first SIM / make SIM state consistent

[inthewaves]

Falling back to first SIM can cause UI mismatch with state. The route from Start new conversation screen avoids this in #147 by setting SIM explicitly, but conversations can be started without going through that flow

Reproduce:

1. In OS Settings > Network and internet > SIMs, under Your primary SIMs for Texts, choose the second SIM

2. Launch new conversation (replace +15551234567 with own number). Make sure messages doesn't have a conversation for this number already

   adb shell 'am start -a android.intent.action.SENDTO -d "smsto:+15551234567" --es sms_body "SIM intent launch test"'
   

3. UI displays Sending with <first sim>, but actual sends will use the <second sim>, i.e. the OS's default SIM for Texts

Originally posted by @RankoR in #125 (comment)