Update hardcoded references to android_*stubs_current

The hardcoded references should be updated to variables exported by
Soong. This enables the stub selection logic to exist in a single place.

Test: TH
Test: No change in out/build-<lunch_product>.ninja

Change-Id: I153f1c6ffc6c93d8a64c3edfef3e8d52c84207a3