Cleanup hardcoded references to android_*stubs_current

These hardcoded refs will need to be updated when we start using .txt
stub equivalent (single-tree/multi-tree). Instead of strewing this logic
all over the codebase, create a helper function that contains the
replacement logic. All other places should call this helper function
instead of calculating the name of .txt equivalent soong module by
itself.

(Will do a similar cleanup in build/make)

Test: no change in ninja file

Change-Id: I6bf999eb4aeaba6ac2a44b9016bae4ec8c79ce19