Make the bazel sandwich ignore targets that don't exist

So that things can be allowlisted that aren't in all branches, or
that aren't present in tests.

Then remove the example partition from the soong integration tests
so that we don't need to pull all of the tools needed to build
partitions into that test.

Bug: 265127181
Fixes: 294220392
Test: ./build/soong/tests/run_integration_tests.sh and m bazel_sandwich
Change-Id: I66fb1ad1c2eee815bfb9700595a26e74e54c6214