Download percentage fluctuates when using an unreliable internet connection

### Expected behavior

The download percentage should only go up if the file successfully downloads a chunk of data

### Actual behavior

The percentage keeps going up and down

### Steps to reproduce the behavior

- Try using a bad WiFi network which cuts off every now and then (a user reported it happening when using WiFi on a train)
- Try downloading a Podcast

https://user-images.githubusercontent.com/16253818/223702575-17618cdc-2f61-43d0-98e1-e0cda1fc41f1.mp4

##### Tested on [iPhone 14], iOS [16.3.1], Pocket Casts [7.32.1]

Reproduce the podcast download on an unreliable WiFi connection, then trace the download-progress handling in the iOS app. Confirm that progress does not decrease when a connection interruption occurs, and add coverage or verification showing the percentage only advances after successful data is received.

Written by the indexing model from the issue text.