Moving platform check for GAE_STANDARD before checking for GCE to avoid a network call

[msjcloud]

Moving check for GAE_STANDARD before checking for GCE.
In GAE Standard environment occassional timeouts were seen, when accessing _METADATA_SERVER_URL
With this change, GAE Standard environment does not have to pay penalty of a network call.

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[msjcloud]

@googlebot I signed it!