Correct bp2build canonical_from_root logic

The logic was inverted from what it should have been,
canonical_from_root --> no strip_import_prefix attribute.

Test: build/bazel/ci/bp2build.sh
Test: build/bazel/ci/mixed_droid.sh
Change-Id: Ic6685d8f0b88279d4444bab3b5e03a544d225f77