Allowlist apexer for bp2build

This also introduces a workaround for the fact that
apexer depends on aapt2, but aapt2 doesn't build
with bp2build yet. Aapt2 is removed from apexer's
requirements during bp2build.

Bug: 204244290
Test: ./build/bazel/ci/bp2build.sh
Change-Id: I837597ce035c7d5c06e1a3957166583a7a94b5c7