Remove direct bitcoin calls from qt/splashscreen.cpp

Summary:
Partial backport of Core PR 10244
https://github.com/bitcoin/bitcoin/pull/10244/commits/5fba3af21e44ab7552c57782de430c1f4cfd6697
Progress towards T417
Depends on D2816, D2842

Test Plan:
make check
ninja
bitcoin-qt # sanity checking

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2817