Fix kotlin nullable errors in CredentialManager

Fix kotlin nullable errors that were exposed by setting the retention
of android.annotation.NonNull and android.annotation.Nullable to
class retention.

Bug: 294110802
Test: builds
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:ea28a7bfcbc297d71c2660632f7352fa9ec8423f)
Merged-In: I9d45c775f6687a30e6e85e9ec3d9fc1944a2dd83

Change-Id: I9d45c775f6687a30e6e85e9ec3d9fc1944a2dd83