Notifications are no longer requested again after rotating the phone

[Isaac-75]

What is it?

• Bugfix (user facing)
• Feature (user facing)
• Codebase improvement (dev facing)
• Meta improvement to the project (dev facing)

Description of the changes in your PR

• Added boolean "notificationsAccepted" field in the App class
• Added getNotificationsAccepted() method, which returns the field
• Added setNotificationsAccepted() method, which sets the field to true
• Incorporated methods in the checkPostNotificationsPermission() method within the PermissionHelper class

Fixes the following issue(s)

• Fixes "allow NewPipe to send you notification" message appears again when changing from portait to landscape view #12194

APK testing

The APK can be found by going to the "Checks" tab below the title. On the left pane, click on "CI", scroll down to "artifacts" and click "app" to download the zip file which contains the debug APK of this PR. You can find more info and a video demonstration on this wiki page.

Due diligence

• [ X] I read the contribution guidelines.

[TobiGr]

Thank you.