Consider the existing input sources MRU only valid if it contained more than one element to pick from. Fixes the following situation with initial-setup sessions:
If we consider the intermediate single-element MRU list invalid, it is still possible to pick the best default source between all the configured ones (the one that was put first in the list, basically), after initialization is complete.
But also, it is unnecessary to have if there is a single source to pick from. After the sources list has two elements of more, the MRU list will become effective and preserved during changes to the available sources.
Closes: #5873 (closed)