Fix data race and ordering consistency in apex modules

apexDepsMutator can be called on multiple apex modules in parallel,
and then two goroutines could call BuildForApex on the same module
in parallel, leading to a data race appending to apexVariations.
This also results in random ordering of the entries in
apexVariations.

Hold a mutex around appending to apexVariations, and sort it before
passing it to ctx.CreateVariations.

Fixes: 134425751
Test: m nothing
Change-Id: If5a3b53a778daacb3e26ac05cde872cf8eb980b3