In https://github.com/mozilla/pdf.js/pull/13661 the API for retrieving
PDF fingerprints (aka. File Identifiers) changed. In generic builds of
PDF.js the old API remains available, but not in the non-generic one
that Firefox's built-in viewer uses.

This commit makes Hypothesis use the new API if available or the old API
otherwise. The fingerprint value should be the same in both cases.

Fixes #3673