[Data collection] Improve UX for "draw an area" flow

Notes from UX meeting:

- [ ] Add a point counter at the bottom left screen denoting number of added points.
- [ ] Drop zone for last point (denoting the complete)
- [ ] Color of last polyline could be different.
- [ ] Last edge should be updated when the map is moving and not when it stops

_Originally posted by @shobhitagarwal1612 in https://github.com/google/ground-android/issues/1351#issuecomment-1507040975_