Bigtable: Validate gc policy top level

[modular-magician]

isTopeLevel in getGCPolicyFromJSON() needs to be passed along correctly. In case of a recursive call, isTopeLevel can be false.

This doesn't cause any problem currently based on the code in validateNestedPolicy(), but could cause a problem later in the future if some logic that depends on isTopeLevel is added.

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bigtable: passed `isTopeLevel` in getGCPolicyFromJSON() instead of hardcoding it to true.

Derived from GoogleCloudPlatform/magic-modules#6427