Variables: Fixes Constant variable persistence confusion (#29407)

* Variables: Fixes savequery for Constant and TextBox variables

* Refactor: reverts textbox changes

* Refactor: Fixes dashboard export and tests

* Refactor: hides or migrates Constant variables

* Tests: fixes snapshots

public/app/features/dashboard/components/DashExportModal/DashboardExporter.ts

@@ -182,16 +182,17 @@ export class DashboardExporter {
               name: refName,
               type: 'constant',
               label: variable.label || variable.name,
-              value: variable.current.value,
+              value: variable.query,
               description: '',
             });
             // update current and option
             variable.query = '${' + refName + '}';
-            variable.options[0] = variable.current = {
+            variable.current = {
               value: variable.query,
               text: variable.query,
               selected: false,
             };
+            variable.options = [variable.current];
           }
         }