Populate initial value from dropdown list for questionnaire items with type reference

[LZRS]

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

Fixes #1830, and close #1831

Description
Adds support for setting initial values using initial expressions for questionnaire field items of type reference in the following forms

Using Resource.id expression

   },
        {
          "url": "http://hl7.org/fhir/uv/sdc/StructureDefinition/sdc-questionnaire-initialExpression",
          "valueExpression": {
            "language": "text/fhirpath",
            "expression": "Patient.id"
          }
        }

and Using Resource

       {
          "url": "http://hl7.org/fhir/uv/sdc/StructureDefinition/sdc-questionnaire-initialExpression",
          "valueExpression": {
            "language": "text/fhirpath",
            "expression": "Patient"
          }
        },

Also adds fix for the autocomplete textview being set to empty when a dropdown item (using the choiceColumn) is selected. Seems to have been occuring when the displayString no longer evaluated to a unique string and .singleOrNull() always evaluated to null

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

Type
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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).

[omarismail94]

@LZRS Can you add a description of what this change does in the Description section above?

[LZRS]

@omarismail94 I've added the relevant description and addressed the comments

[omarismail94]

Great change!

[omarismail94]

Nice catch! Good to see our testing also caught it :D

[jingtang10]

slightly worried we're not dealing with identify of codes carefully enough. can we make this function much more rigorous please?

for example, comparing two coding just by code but not by system doesn't meet the brief of returning "unique identifier".