[Firefox] Only load 'PdfStreamConverter' and 'PdfRedirector' when PDF.js is enabled

This PR moves the import of `PdfStreamConverter` and `PdfRedirector` inside of the `_ensureRegistered` funtion in PdfJs.jsm, to avoid loading them if PDF.js is disabled.

Note that this is already done for the addon, so this change makes the `mozcentral` and `firefox` implementations behave the same way.