[Survey list] Simplify UI state design

* `uiState` is serving as both a state and an event to represent completion of survey activation or errors.
* The list, being reactive, updates once a survey is made available offline. This causes the state to change, leading to workarounds in https://github.com/google/ground-android/pull/2768. This can be simplified by splitting states and events as in the previous point.

@anandwana001 @shobhitagarwal1612 @scolsen FYI