text: Title: [APM] Improve function name: getIsUsingTransactionEvents
  Body: In order to show the callout, the notifies the user when they are using transaction data instead of metrics we create a function called [getIsUsingTransactionEvents](https://github.com/elastic/kibana/blob/master/x-pack/plugins/apm/server/lib/helpers/aggregated_transactions/get_is_using_transaction_events.ts#L15). But this name doesn't match what this function returns, e.g. when `SearchAggregatedTransactionSetting is never` all data is queried from the transaction index, but this function returns false.
  
  We could call it `should_display_fallback_message` or open to suggestions.
  
  Also, there's an opportunity to combine some of the logic from [`getSearchAggregatedTransactions` ](https://github.com/elastic/kibana/blob/master/x-pack/plugins/apm/server/lib/helpers/aggregated_transactions/index.ts#L53): While `getIsUsingTransactionEvents` depends on `getSearchAggregatedTransactions` while both are making similar checks on the `xpack.apm.searchAggregatedTransactions` config value. In code path, calling `getIsUsingTransactionEvents` may call `getSearchAggregatedTransactions` which runs an ES query, then calls `getHasTransactions` which runs another ES query. We should avoid this if possible.