active call notification must not be clearable

### Steps to reproduce

1. start a call
2. clear call notification

### Outcome

#### What did you expect?

The call notification must not be possible to clear during the call.

#### What happened instead?

Notification disappears, call continues.

### Your phone model

_No response_

### Operating system version

_No response_

### Application version and app store

0.7.3

### Homeserver

_No response_

### Will you send logs?

No

### Are you willing to provide a PR?

No

No file, test, or entry point is named. Start by reproducing the issue on an Android device with version 0.7.3, then locate the active-call notification handling and check its behavior while a call is ongoing. Done means clearing the notification during a call no longer removes it, while the call continues normally.

Written by the indexing model from the issue text.