ConfigUpdateInstallReceiver: pass content via content provider

When receiving a request to update ConfigUpdater content, don't
try to open by filename. Fetch the content via a file descriptor
delivered from a content provider. This avoids the files needing to
be marked world-readable, and resolves an SELinux violation.

Bug: 14989241
Change-Id: I10ad0d710c9a833a45995c545ba585a533c35b0d