Show progress indicator when downloading the `vault.cryptomator`-file so that the user knows that the app is preparing to unlock the vault

### Please agree to the following

- [X] I have searched [existing issues](https://github.com/cryptomator/android/issues?q=) for duplicates
- [X] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/android/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

Show progress indicator while downloading the `vault.cryptomator` file

### System Setup

```markdown
- Android: x
- Cryptomator: 1.6.8
```

### Cloud Type

_No response_

### Steps to Reproduce

1. Have a poor internet connection, e.g. open emulator settings, switch in Cellular `Network type` to `GPRS` and `Signal strength` to `Poor`, disable Wi-Fi
3. Unlock vault

### Expected Behavior

A progress indicator is displayed.

### Actual Behavior

The progress indicator is displayed only while unlocking the vault (and downloading the `masterkey.cryptomator`-file) but not while getting the `vault.cryptomator`-file which leads to the user thinking that the app does nothing and the click on the vault was somehow unsuccessful or a silent error occurred:

https://user-images.githubusercontent.com/1786772/152775204-64f1208f-3904-44cf-98f0-7022ec4cb31e.mp4

### Reproducibility

Always

### Relevant Log Output

_No response_

### Anything else?

_No response_