DPC-620 Preloaded subscription are not applied

In case of failed download, (no internet connection) FilterListContentProvider was returning filters file that contained only header:

[Adblock Plus 2.0]
! This file was automatically created.

https://gitlab.com/eyeo/distpartners/adblockplussbrowser/-/blob/dev/core/src/main/kotlin/org/adblockplus/adblockplussbrowser/core/provider/FilterListContentProvider.kt#L136

https://gitlab.com/eyeo/distpartners/adblockplussbrowser/-/blob/dev/core/src/main/kotlin/org/adblockplus/adblockplussbrowser/core/work/UpdateSubscriptionsWorker.kt#L245

This useless file needs to be deleted. So, we are not entering into this if: https://gitlab.com/eyeo/distpartners/adblockplussbrowser/-/blob/dev/core/src/main/kotlin/org/adblockplus/adblockplussbrowser/core/provider/FilterListContentProvider.kt#L134

BTW Preloaded subs have been updated.

Closes DPC-620