GULHeartbeatStorage: use user defaults instead of a file on tvOS

[maksymmalyhin]

The cache directory (the only available for apps on tvOS) may be erased at any time, so is not a reliable place to store heartbeats. User defaults should be used instead.

Related PR: https://github.com/firebase/firebase-ios-sdk/pull/6658/files#diff-ba0d6f67c0194dcbcd6a90cfd4ee2a03R61