Restrict validateApexClasspathFragments to active apex prebuilts

This is a followup to https://r.android.com/3073624 that moved
validation of bcp jars to the top-level prebuilt apex. It is possible
that there can be multiple prebuilt variants of a mainline module and
each prebuilt might have a skew in bcp jars. This CL limits the check to
only the selected apex prebuilt.

Test: go test ./apex
Bug: 328578801
Change-Id: If225a1af6004b6584b86ec442f99672f0d1f8314