Non-uniform functionality of back button for different activities

### Actual behaviour

Currently, if we press back button while on a particular activity, the back functionality for different set of activities is different.

### Expected behaviour

The behaviour of back button should be uniform across all activities. All activities should direct us to the home activity i.e. the All files activity on pressing back button.

### Steps to reproduce
1. Open the app and log into your account.
2. Open Activities from the navigation drawer. Then press back button. This takes us to the All files activity.
3. Open Favourites/Photos/Shared/On Device activity. Then press back button. This causes exit from the app.
4. Open Notifications/Auto Upload/Uploads/Settings/Participate activity. Then press back button. This takes us to the On Device activity.

### Environment data
Android version: 7.0

Device model: Xiaomi Redmi Note 4