fix(service-worker): detect new version even if files are identical to an old one (#26006)

Previously, if an app version contained the same files as an older
version (e.g. making a change, then rolling it back), the SW would not
detect it as the latest version (and update clients).

This commit fixes it by adding a `timestamp` field in `ngsw.json`, which
makes each build unique (with sufficiently high probability).

Fixes #24338

PR Close #26006

packages/service-worker/config/test/generator_spec.ts

@@ -10,6 +10,8 @@ import {Generator} from '../src/generator';
 import {MockFilesystem} from '../testing/mock';
 
 describe('Generator', () => {
+  beforeEach(() => spyOn(Date, 'now').and.returnValue(1234567890123));
+
   it('generates a correct config', done => {
     const fs = new MockFilesystem({
       '/index.html': 'This is a test',
@@ -70,6 +72,7 @@ describe('Generator', () => {
     res.then(config => {
          expect(config).toEqual({
            configVersion: 1,
+           timestamp: 1234567890123,
            appData: {
              test: true,
            },
@@ -137,6 +140,7 @@ describe('Generator', () => {
     res.then(config => {
          expect(config).toEqual({
            configVersion: 1,
+           timestamp: 1234567890123,
            appData: undefined,
            index: '/test/index.html',
            assetGroups: [],