Remove dependencies on the 1-variant fallback

Currently, adding depedencies will use the only variant of a module
if only 1 variant exists. Otherwise, the variations of the two modules
+ the variations explicitly requested must match. The 1-variant
fallback causes issues for incremental soong, so remove reliances on it.

Bug: 372091092
Test: m nothing --no-skip-soong-tests
Change-Id: I778d1d8d67e9d0ccc3432f12e50d15c174b5dbf5