TimeCreditShopActivity can be created multiple times on top of each other

[leonpoint]

Describe the bug
When the TimeCreditShopActivity is being started multiple times, it takes multiple presses on the back button to get back to the app

To Reproduce
Steps to reproduce the behavior:

1. have an app be blocked by the learning aid (branch dh-7-applock)
2. repeatedly try to open the blocked app
3. the app will be blocked and the timecreditshop will open

alternatively to 1-3 you can just start multiple intents of the shopactivity to test it
4. open the Uniatron app
5. press the back button and watch as you have to close multiple instances of the shop activity

Expected behavior
the shop should close after pressing back once