current leg index incorrect after reroute for multi-leg routes

Nobody has claimed this yet.

Capturing from @tkvalentine56 in https://github.com/mapbox/mapbox-navigation-android/pull/4999#issuecomment-953972363:
I have a 25 waypoint route which navigates fine until an OFF ROUTE state is experienced. When the route progress event state changes back to TRACKING the current leg index is always zero and the route leg count increments by one. I am then directed to continue navigation to the the first point on the route (which is now the additional point added which appears to be the original first point). This ultimately produces a navigation experience which is confusing, illogical and un-useful.

What I expect after transitioning from OFF ROUTE to TRACKING is that the current leg index be that of the next unarrived waypoint on the route and the route leg count to remain the same.

Here is log output from a navigation run today. Notes are embedded as well as the route progress event associated route directions json.
[route_directions_log_output.txt](https://github.com/mapbox/mapbox-navigation-android/files/7455820/route_directions_log_output.txt)

Start with route_directions_log_output.txt and the route progress events described in the report, then trace how OFF ROUTE transitions back to TRACKING update the current leg index and route leg count. Done means a multi-leg route preserves its leg count and resumes at the next unarrived waypoint after rerouting, with regression coverage for the reported case.

Written by the indexing model from the issue text.