The collada export code was directly using `action->curves` in its
export code, which doesn't work with slotted actions. This commit
updates that code to use wrapper functions that provide access to the
correct fcurves regardless of whether the action is slotted or not.
Note that the import code has not yet been updated, and is left for
a future PR.
Ref: #123424
Pull Request: https://projects.blender.org/blender/blender/pulls/128061
629b3ccd42