Default apex_available to //apex_available:platform

This means everything that goes into apexes need to be
explicitly labeled to be available for apex.

Whitelist the current offenders. This list should be
trimmed down.

Bug: 147364041
Test: m
Test: multiproduct_kati -only-soong
Exempt-From-Owner-Approval: cherry-pick from internal
Change-Id: I837299c6a15d46f8a5ba544b613776b1cc27d7b8
Merged-In: I837299c6a15d46f8a5ba544b613776b1cc27d7b8
(cherry picked from commit 93488cbb10d4882845abb732f8e53714f0982031)