[CredMan] Unhide the getPendingCredential api.

Per partner feedback, expose a version of getCredential api that splits
prefetching candidates and selection UI into two separate calls. This
allows performing prefetch work and an early time and delaying the UI to
a later time which will be significantly faster to surface upon request.

Bug: 273308895
Test: CTS & local tested
Change-Id: I2f08a4333ac90bb9fdc77cb8f522b415cd694336