Refactor the DownloadManager interface

[jingtang10]

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

Description
Refactor the DownloadManager interface:

• Renames it to DownloadWorkManager to signify that it only handles a single "download job" and needs to be reset/recreated before the next download job (to be clarified).
• Makes the handling of the URL an internal feature of the DownloadWorkManager. In other words, the DownloadWorkManager acts as an iterator that provides the URL for the next download request.
• Combines extractResourcesFromResponse and extractNextUrlsFromResource into a single processResponse function.
• Extracts the createDownloadUrl function as a utility function outside of the interface because it is something that is universal to FHIR requests and not tied down to any single implementation of the `DownloadWorkManager.

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

Type
Code health

Screenshots (if applicable)

Checklist

• I have read and acknowledged the Code of conduct.
• I have read the Contributing page.
• 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 demo app(s) to verify my change fixes the issue and/or does not break the demo app(s).

[aditya-07]

LGTM

[codecov]

Codecov Report

Merging #1266 (7585131) into master (a292e50) will decrease coverage by 0.27%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1266      +/-   ##
============================================
- Coverage     84.15%   83.87%   -0.28%     
  Complexity      666      666              
============================================
  Files           146      146              
  Lines         10609    10605       -4     
  Branches        807      838      +31     
============================================
- Hits           8928     8895      -33     
+ Misses         1278     1277       -1     
- Partials        403      433      +30     

Impacted Files	Coverage Δ
...src/main/java/com/google/android/fhir/sync/Sync.kt	23.80% <ø> (ø)
...n/java/com/google/android/fhir/sync/SyncJobImpl.kt	53.48% <ø> (-4.66%)	⬇️
...ava/com/google/android/fhir/sync/FhirSyncWorker.kt	93.47% <100.00%> (ø)
...a/com/google/android/fhir/sync/FhirSynchronizer.kt	79.31% <100.00%> (-3.45%)	⬇️
...oogle/android/fhir/sync/download/DownloaderImpl.kt	100.00% <100.00%> (ø)
...alidation/PrimitiveTypeAnswerMinLengthValidator.kt	70.00% <0.00%> (-20.00%)	⬇️
...com/google/android/fhir/datacapture/DataCapture.kt	66.66% <0.00%> (-16.67%)	⬇️
...main/java/com/google/android/fhir/DatastoreUtil.kt	80.00% <0.00%> (-10.00%)	⬇️
...roid/fhir/datacapture/validation/RegexValidator.kt	85.71% <0.00%> (-7.15%)	⬇️
...java/com/google/android/fhir/resource/Resources.kt	53.33% <0.00%> (-6.67%)	⬇️
... and 15 more

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