Avoid using Java 8 `ConcurrentHashmap.newKeySet()`

[naveensingh]

As it can cause NoSuchMethodError crashes because of https://stackoverflow.com/questions/64506917/using-concurrenthashmapnewkeyset-with-android-corelibrarydesugaring.

This won't affect MediaStore scan performance so it's probably not worth the time to investigate it further.