Regression in navigation when returning to home tabs after discarding observation flow

[achou11]

Surfaced in #257, but the regression can be reproduced with the following steps:

1. Navigate to camera tab
2. Press create observation button
3. Press button in top-left to discard observation (i.e. leave observation creation flow)
4. Returns to map screen

The result in step (4) is different than what happens in Mapeo Mobile. In Mapeo Mobile, the app returns to the camera tab in step (4). I don't expect the new behavior to be the intention, so I'm classifying this as a regression