[Spa] Add Scaffold isFirstLayerPageWhenEmbedded param

And if the page is first layer page when embedded and the activity is
embedded, not show the Scaffold's back button, because it is the first
layer page now.

Bug: 408233278
Flag: EXEMPT bug fix
Test: manual - with gallery
Change-Id: Ifd37d19061d805ef87c7a2ae76c5c2e5af7aa55f