Change Identity Service impl to depend only on signin internals

Now that a distinct GN target exists just for the code backing
IdentityManager/Identity Service, change the Identity Service
implementation to depend only on that target. The change worked right
out of the box -- good validation of the initial split.

Bug: 901859
Change-Id: I9b04ef952c34d9e9c0efcf137c55d41c8d616aff
Reviewed-on: https://chromium-review.googlesource.com/c/1323649
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606806}

services/identity/BUILD.gn

@@ -18,7 +18,7 @@ source_set("lib") {
 
   deps = [
     "//base",
-    "//components/signin/core/browser",
+    "//components/signin/core/browser:internals",
     "//components/signin/core/browser:shared",
     "//services/identity/public/cpp:cpp_types",
     "//services/identity/public/mojom",
@@ -39,9 +39,9 @@ source_set("tests") {
     "//base",
     "//base/test:test_support",
     "//components/prefs:test_support",
-    "//components/signin/core/browser",
+    "//components/signin/core/browser:internals",
+    "//components/signin/core/browser:internals_test_support",
     "//components/signin/core/browser:shared",
-    "//components/signin/core/browser:test_support",
     "//components/sync_preferences:test_support",
     "//mojo/public/cpp/bindings:bindings",
     "//services/identity/public/cpp",