Launch the CredentialManager UI from the app context.

This is a temporary version that does not impact public apis. A
follow-up change will be sent out that adds the api changes and properly
launches the UI using an app activity, not just the context.

Test: deployed locally.
Bug: 246564035
Bug: 253156924
Bug: 253156958
Change-Id: I3f43fb7332d58f2baffab7a80c4550d95c67854e