Allow dependencies from platform variants to APEX modules.

When `test_for` dependencies are added from libraries to APEX modules,
they can be created from the platform variants of the libraries, since
those are used for building tests. Hence we need an alias from the
platform variant of the APEX module to have a target for those
dependencies.

This is only necessary for libraries that are split by the APEX
mutator, i.e. is a member of some APEX. Normally that's not the case
for test libraries, but there may be exceptions (read
com.android.art.testing).

Test: m nothing
Bug: 183882457
Change-Id: I68affdf69d7ec05c0ee8730e8ec04d7cb9e0e44a