mockgcp: Fix return type for mockapikeys

[justinsb]

I believe two PRs "crossed in the mail", and thus the interface method
has the wrong return type.

[google-oss-prow]

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[gemmahou]

Duplicate of #1194

[justinsb]

Looks like this was fixed in #1194 (thanks!)

/close

[google-oss-prow]

@justinsb: Closed this PR.

In response to this:

Looks like this was fixed in #1194 (thanks!)

/close

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[maqiuyujoyce]

Just curious - why was the issue not detected by the presubmit test in #1168 ?