apex: Make android_library support APEX variants

The android_library module type can be used within an android_app that
can itself be added to an apex but android_library does not call
android.InitApexModule(module) and so it does not support apex variants
and so is treated as if it cannot be part of an APEX even though it
actually is.

Now, android_library supports AEPX variants.

Due to this change, the whilelist for apex_available got bigger.

Exempt-From-Owner-Approval: Got approval in rvc-dev.

Bug: 156978407
Test: m
Change-Id: I3a21d653fe9c4159e3a89791fe1a8597865eeae6