text: Title: [APM] Use `ProcessorEvent` constants everywhere instead of literals
  Body: We have a `ProcessorEvent` enum:
  https://github.com/elastic/kibana/blob/d1de029b8873e29c35df4bf6ce95f987efd83874/x-pack/legacy/plugins/apm/common/processor_event.ts#L7-L12
  
  This should be used instead of string literals:
  ```js
  // bad
  term: { [PROCESSOR_EVENT]: 'transaction' }
  
  // good
  term: { [PROCESSOR_EVENT]: ProcessorEvent.transaction }
  ```
  
  **Examples where `ProcessorEvent` is used:**
  https://github.com/elastic/kibana/blob/d1de029b8873e29c35df4bf6ce95f987efd83874/x-pack/legacy/plugins/apm/server/lib/transactions/avg_duration_by_browser/fetcher.ts#L30
  
  **Examples where `ProcessorEvent` should be used:**
  https://github.com/elastic/kibana/blob/d1de029b8873e29c35df4bf6ce95f987efd83874/x-pack/legacy/plugins/apm/common/projections/services.ts#L34
  
  https://github.com/elastic/kibana/blob/d1de029b8873e29c35df4bf6ce95f987efd83874/x-pack/legacy/plugins/apm/server/lib/index_pattern/get_dynamic_index_pattern.ts#L81-L83
  
  https://github.com/elastic/kibana/blob/d1de029b8873e29c35df4bf6ce95f987efd83874/x-pack/legacy/plugins/apm/server/routes/index_pattern.ts#L28-L30
  
  ... and many other places.