Replace usage of BroadcastChannel with SharedFlow

[twyatt]

🏗️ Changes

• KeepAliveGatt uses underlying Gatt's onConnectionStateChange data stream to detect connection drop
  • Previously relied on closure of onCharacteristicChanged BroadcastChannel
• Event handler (e.g. onConnected, onDisconnected, etc) exceptions are propagated to collector scope
  • Was previously propagated to KeepAliveGatt's parent scope
• Reconnect (via disconnect()) and disconnect (via keepAliveGatt.disconnect()) are possible directly from event handler functions

📓 Notes

Since SharedFlow is not yet officially released, this branch was developed against internally built Coroutines artifacts; this PR will be merged into internal branch (see #86 for details).

[cedrickcooke]

Plz re-request review when out of draft

[twyatt]

Small update: This PR was brought into develop via a fast-forwarding of develop to internal (that was dropped). So some slight modifications of the squashed commit may exist in the final commit that landed in develop.