feat: Add reportAppFullyDisplayed to RUM for app launch time to full display (TTFD) - #1124

• All applicable files are owned properly
  _{Digest: 7568a54}

• All required reviewers must approve this pull request before it can be merged. Learn more in our FAQ.

• Needs approval from Team: rum-mobile

  _{Questions about this check? Reach out in #dx-source-code-management on Slack.
  Digest: f27730e}

if this seems incorrect, you can retrigger the evaluation of the MergeGate by clicking on this link