Use a PayDM mock in CreditCardSaveManagerTest

The CreditCardSaveManagerTest currently mocks the PDM, which is the
reason why five function in the PDM still forward calls to the PayDM.
This CL replaces this PDM mock with a PayDM mock and removes the
affected functions from the PDM.
It does so by introducing setters for the ADM and PayDM in the TestPDM.

Bug: b/322170538
Change-Id: If6b61aeb3794c419504f7fd2e0eed6bfa6d2aa6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5435459
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Cr-Commit-Position: refs/heads/main@{#1284429}