Added code to generate TokenIndex for fhirType code

[aditya-07]

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Fixes : NO issue created

Description
Searchable fields like Patient.gender were not getting indexed in the database.

Alternative(s) considered
Have you considered any alternatives? And if so, why have you chosen the approach in this PR?

Type
Choose one: Feature

Screenshots (if applicable)

Checklist

• I have read and acknowledged the Code of conduct
• I have read How to Contribute
• I have read the Developer's guide
• I have signed the Google Individual CLA, or I am covered by my company's Corporate CLA
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach
• I have run ./gradlew spotlessApply and ./gradlew spotlessCheck to check my code follows the style guide of this project
• I have run ./gradlew check and ./gradlew connectedCheck to test my changes locally
• I have built and run the reference app(s) to verify my change fixes the issue and/or does not break the reference app(s)

[codecov-commenter]

Codecov Report

Merging #693 (39fe9a5) into master (2ccda9b) will decrease coverage by 0.24%.
The diff coverage is 0.00%.

❗ Current head 39fe9a5 differs from pull request most recent head 9a1b460. Consider uploading reports for the commit 9a1b460 to get more accurate results

@@             Coverage Diff              @@
##             master     #693      +/-   ##
============================================
- Coverage     58.02%   57.78%   -0.25%     
  Complexity      194      194              
============================================
  Files            45       45              
  Lines           891      893       +2     
  Branches        122      124       +2     
============================================
- Hits            517      516       -1     
  Misses          295      295              
- Partials         79       82       +3     

Impacted Files	Coverage Δ
...a/com/google/android/fhir/index/ResourceIndexer.kt	46.93% <0.00%> (-0.65%)	⬇️
...va/com/google/android/fhir/db/impl/DatabaseImpl.kt	35.55% <0.00%> (-2.23%)	⬇️

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