Enforce stub libraries should have a single apex_available

If a library contributes to an API surface, it will have only a single
copy on device. Therefore, we should disallow installation to muliple
apexes/platform.

There are some exceptions to this rule today, and they have been relaxed
using allowlists.

Bug: 277651159
Test: go test ./apex
Change-Id: Ice3023ecd28412a2610d8b98628cb727b58c5c3b