Bug 1741600 - Fix ExpandedPrincipal and nsCSPContext leaks triggered by ApplyAddonContentScriptCSP. r=ckerschb,robwu

This patch is meant to be a proposed short run fix to prevent ApplyAddonContentScriptCSP
from leaking the ExpandedPrincipal and nsCSPContext instance because they keep a reference
to each other.
This patch prevent that leak by creating a clone of the ExpandedPrincipal and then use
that cloned instance in the call to nsCSPContext::SetRequestContextWithPrincipal.

Once Bug 1548468 will move the CSP off the ExpandedPrincipal class, cloning the expanded
principal to prevent that leak should not be necessary anymore.

Differential Revision: https://phabricator.services.mozilla.com/D132144