Display Froyo-compatible notification when scanning starts

This PR fixes #27.

1. The `Notification.Builder` code is only supported in API Level 11+. This PR creates the notification using a (deprecated) `Notification` constructor instead of `Notification.Builder`.
2. This PR also displays the notification icon when scanning starts, not when `MainActivity.onStop()` is called. Displaying the notification immediately visually ties the notification's lifetime with the scanning period.
3. The new notification makes the toasts redundant, so I removed the calls (but left the toast code so we can use it for other service alerts).