Revert "refactor: use isObservable provided by rxjs 6.1+ (#27668)"

This reverts commit 7e6350ce2e.

packages/core/src/render3/instructions/listener.ts

@@ -6,8 +6,9 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {isObservable} from 'rxjs';
+
 import {assertDataInRange} from '../../util/assert';
+import {isObservable} from '../../util/lang';
 import {EMPTY_OBJ} from '../empty';
 import {PropertyAliasValue, TNode, TNodeFlags, TNodeType} from '../interfaces/node';
 import {GlobalTargetResolver, RElement, Renderer3, isProceduralRenderer} from '../interfaces/renderer';