Generalize the mechanism on how to setup appContext

- Don't rely on a global for the initialization logic
- Generalize the logic to support any Context type

Google-Bug-Id: b/206459178
Change-Id: I3ce49e38a17bdca144dfb794625b4153a275315e
diff --git a/polygerrit-ui/app/elements/admin/gr-admin-group-list/gr-admin-group-list.ts b/polygerrit-ui/app/elements/admin/gr-admin-group-list/gr-admin-group-list.ts
index 83063a1..afdb38b 100644
--- a/polygerrit-ui/app/elements/admin/gr-admin-group-list/gr-admin-group-list.ts
+++ b/polygerrit-ui/app/elements/admin/gr-admin-group-list/gr-admin-group-list.ts
@@ -30,7 +30,7 @@
 import {GroupId, GroupInfo, GroupName} from '../../../types/common';
 import {GrCreateGroupDialog} from '../gr-create-group-dialog/gr-create-group-dialog';
 import {fireTitleChange} from '../../../utils/event-util';
-import {appContext} from '../../../services/app-context';
+import {getAppContext} from '../../../services/app-context';
 import {SHOWN_ITEMS_COUNT} from '../../../constants/constants';
 
 declare global {
@@ -91,7 +91,7 @@
   @property({type: String})
   _filter = '';
 
-  private readonly restApiService = appContext.restApiService;
+  private readonly restApiService = getAppContext().restApiService;
 
   override connectedCallback() {
     super.connectedCallback();