Do not start host in "offline mode" if app is "read-only"

We start the host in "offline mode" if the host detects an `app_offline.htm` file. This relies on the host getting restarted whenever the app_offline.htm is removed. HOWEVER, if the app is initialized in read-only mode, it may never switch back to being online (related: https://github.com/Azure/azure-functions-host/issues/4387).

cc: @fabiocav

The issue names no files or tests; start by tracing how app_offline.htm detection interacts with read-only initialization, using related issue #4387 for context. Done means a read-only app does not remain in offline mode after app_offline.htm is removed.

Written by the indexing model from the issue text.