Add apex_available platform to CtsShimPrivUpgrade

This fixes the CTS shim build. While it's not a perfect solution
(CtsShimPrivUpgrade shouldn't be available to platform), it's easy and
works and doens't introduce any risks - if a device product starts to
include CtsShimPrivUpgrade in their system image, they will fail the CTS
tests.

We can look into a proper solution in U timeframe when we have more
time.

Bug: 230724040
Test: vendor/google/cts/shim/build_all_shim.sh
Change-Id: Icf71e9d8a7b8c7983f19fbf8667ead669e662b5b