Refactor how managed mode and provider are determined

### What needs to get done

Snapcraft determines if it should run in managed mode or not and which provider to use. This is done in 2 discrete places in the codebase. This was upstreamed in https://github.com/canonical/craft-application/pull/242.

To make the code more readable and maintainable in craft-application, this code could be combined and done earlier in craft-application's execution.

Source: https://github.com/canonical/craft-application/pull/242#discussion_r1507992182

### Why it needs to get done

To reduce code debt

Start by reading the linked craft-application pull request, especially discussion comment 1507992182, then locate the two execution points where managed mode and the provider are currently determined. Combine that determination earlier in craft-application, and confirm the refactor preserves the existing behavior while reducing the duplicated decision flow.

Written by the indexing model from the issue text.