[Multiple Requests] Fix VCN enrollment issues on Mobile.

On Clank settings page, autofill_client is nullptr and cannot provide
a PaymentNetworkInterface properly. It has to be provided in the
VCEM constructor.

On Bling, the accessor for multiple request payment network interface
returns nullptr due to no implementation.

Bug: 403617982
Change-Id: I9a5627f5b6453a1115f22709a9a2ae3e6f199e9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6762194
Reviewed-by: Olivia Saul <osaul@google.com>
Reviewed-by: Siddharth Shah <siashah@chromium.org>
Commit-Queue: Siyu An <siyua@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1493066}