refactor(core): Ensure that previousOrParentTNode always belongs to current TView. (#38707)

`previousOrParentTNode` stores current `TNode`. Due to inconsistent
implementation the value stored would sometimes belong to the current
`TView` and sometimes to the parent. We have extra logic which accounts
for it. A better solution is to just ensure that `previousOrParentTNode`
always belongs to current `TNode`. This simplifies the mental model
and cleans up some code.

PR Close #38707

packages/core/test/sanitization/sanitization_spec.ts

@@ -20,7 +20,7 @@ function fakeLView(): LView {
 }
 
 describe('sanitization', () => {
-  beforeEach(() => enterView(fakeLView(), null));
+  beforeEach(() => enterView(fakeLView()));
   afterEach(() => leaveView());
   class Wrap {
     constructor(private value: string) {}