Make all_apex_contributions a regular module

It was not using any of the capabilities of a singleton module,
and I think the fact that it's a singleton module is causing issues
for the base configuration change. This is also the last singleton
module in the tree, we might be able to remove them after.

Bug: 361816274
Bug: 370379708
Test: m nothing --no-skip-soong-tests
Change-Id: I6193d77837f2e55f7fb6bc41135c8f82fd506c73