Do not propagate the stub symbol file to prebuilts.

This partially reverts https://r.android.com/1278193. The propagated
symbol files contain all versions, even though only one is applicable
in a given SDK snapshot.

It's uncertain what repercussions this might have, but one is that if
we were to update a snapshot for a fixed version then it might change
because the symbol file contains new versions that aren't applicable.
Since the symbol file isn't actually needed at this point it's better
to wait with this step until the use cases for it are more clear.

Test: m nothing
Test: Create an SDK snapshot with Bionic libs, drop it into a
  master-art tree without bionic/ in it, build ART APEXes, and check
  that the Soong phase completes (specifically that the stubs are
  detected even without symbol files).
Bug: 152481980
Change-Id: Ic79f89bc6d11d0b6552fa20791f5680ff9a40c0d