Use fragments instead of new activities

### Steps to reproduce
1. Open navigation drawer
2. Opening "Activities", "Notifications", "Auto Upload", "Uploads", and "Deleted files"
3.

### Expected behaviour
Opening the feature in the same activity (com.owncloud.android.ui.activity.FileDisplayActivity), just switch the fragment, like opening "Favorites", "Photos", etc.

### Actual behaviour
Opens the feature in a new activity, and shows an animation of opening a new activity.

### Environment data
Android version:
Android 9

Device model:
OnePlus 5 (ONEPLUS A5000)

Stock or customized system:
Stock-like H2OS 9.0.7
(Preinstalled on Chinese models of OnePlus Phones, similar to Oxygen OS)

Nextcloud app version:
3.10.1 from Play Store

Nextcloud server version:
17.0.3

### Logs
#### Web server error log
(Not server related problem.)

#### Nextcloud log (data/nextcloud.log)
(App not crashed, just an user experience enhancement.)