[APM] Synthtrace improvements in mobile

[MiriamAparicio]

While implementing the mobile transactions tables, there were some comments on the PR of things that can be improved in synthtrace mobile data generation

• kibana/packages/kbn-apm-synthtrace-client/src/lib/apm/mobile_device.ts

  Line 118 in 03464e7

  startNewSession() {

  
  Synthtrace shouldn't have side effects like this. We should use an API like .session() which returns a session

• We can refactor and inline the implementation of crashes

transaction.errors( ...(couldCrash && index % 2 === 0 ? [ device.crash(...)] : []) )

• We should try to avoid ES field names when using Synthtrace, it should be an implementation detail, so we can more easily rename stuff or create a different output target, like APM Server instead of ES
  https://github.com/elastic/kibana/pull/153044/files/ec047aa246f8f17850bf1079db4ff4a6d79a7931#diff-232eadeb67c3adf2b059efb3f329c7aa670a0da2f1e400edf5923e6221b1b52c

• Find a way to generate the launch metrics (this might not be relevant as the application launch count it's store in lyfecycle events)
  [APM] Add table tabs showing summary of metrics #153044 (comment)

[elasticmachine]

Pinging @elastic/apm-ui (Team:APM)