fix(ivy): process nested animation metadata (#32818)

In View Engine, animation metadata could occur in nested arrays which
would be flattened in the compiler. When compiling a component for Ivy
however, the compiler no longer statically evaluates a component's
animation metadata and is therefore unable to flatten it statically.
This resulted in an issue to find animations at runtime, as the metadata
was incorrectly registered with the animation engine.

Although it would be possible to statically evaluate the animation
metadata in ngtsc, doing so would prevent reusable animations exported
from libraries from being usable as ngtsc's partial evaluator is unable
to read values inside libraries. This is unlike ngc's usage of static
symbols represented in a library's `.metadata.json`, which explains how
the View Engine compiler is able to flatten the animation metadata
statically.

As an alternative solution, the metadata flattening is now done in the
runtime during the registration of the animation metadata with the
animation engine.

Fixes #32794

PR Close #32818

packages/core/test/animation/animation_integration_spec.ts

@@ -328,6 +328,36 @@ const DEFAULT_COMPONENT_ID = '1';
         ]);
       });
 
+      // https://github.com/angular/angular/issues/32794
+      it('should support nested animation triggers', () => {
+        const REUSABLE_ANIMATION = [trigger(
+            'myAnimation',
+            [transition(
+                'void => *', [style({'opacity': '0'}), animate(500, style({'opacity': '1'}))])])];
+
+        @Component({
+          selector: 'if-cmp',
+          template: `
+          <div @myAnimation></div>
+        `,
+          animations: [REUSABLE_ANIMATION],
+        })
+        class Cmp {
+        }
+
+        TestBed.configureTestingModule({declarations: [Cmp]});
+
+        const engine = TestBed.inject(ɵAnimationEngine);
+        const fixture = TestBed.createComponent(Cmp);
+        fixture.detectChanges();
+        engine.flush();
+
+        expect(getLog().length).toEqual(1);
+        expect(getLog().pop() !.keyframes).toEqual([
+          {offset: 0, opacity: '0'}, {offset: 1, opacity: '1'}
+        ]);
+      });
+
       it('should allow a transition to use a function to determine what method to run', () => {
         let valueToMatch = '';
         let capturedElement: any;