[ios] Add helper methods to MainApplicationDelegate

The block in -firstSceneWillEnterForeground: was a bit difficult to
extend, so extract it to -firstSceneDidEnterForeground method, with
a few other helper methods to compute whether the app was started
via external intent or not, and to add code to execute when started
by an external intent.

Bug: 329639476
Change-Id: I1926d6e7cfeef14ea5ff8ac57ce372d0bcb92277
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5412939


Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281815}