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/test/integration_spec.ts

@@ -31,6 +31,7 @@ function obsToSinglePromise<T>(obs: Observable<T>): Promise<T> {
 
 const manifest: Manifest = {
   configVersion: 1,
+  timestamp: 1234567890123,
   appData: {version: '1'},
   index: '/only.txt',
   assetGroups: [{
@@ -46,6 +47,7 @@ const manifest: Manifest = {
 
 const manifestUpdate: Manifest = {
   configVersion: 1,
+  timestamp: 1234567890123,
   appData: {version: '2'},
   index: '/only.txt',
   assetGroups: [{