Third-Party Import of: R4 FHIR spec

This was downloaded from https://www.hl7.org/fhir/R4/downloads.html,
specifically by expanding the zip file from https://www.hl7.org/fhir/R4/definitions.json.zip

Request Document: go/android3p
For CL Reviewers: go/android3p#reviewing-a-cl
For Build Team: go/ab-third-party-imports
Bug: http://b/361795175
Original import of the code can be found at: https://googleplex-android.googlesource.com/platform/external/fhir/spec/r4/+/refs/heads/third-party-review.
Security Questionnaire: http://b/361795175#comment1
Test: No test. Importing third party json files

Change-Id: I4ed3ab61761182ffd8b1866b47690981feae83e1