Restructure public side of view_manager service.

services/window_manager/BUILD.gn

Index: services/window_manager/BUILD.gn
diff --git a/services/window_manager/BUILD.gn b/services/window_manager/BUILD.gn
index 854fa6814cb41398cfa233b00fa97ef01a49ddc3..ce4353e42c4577755c68f9dfb26147c151ec51da 100644
--- a/services/window_manager/BUILD.gn
+++ b/services/window_manager/BUILD.gn
@@ -18,7 +18,7 @@ mojo_native_application("window_manager") {
     "//base",
     "//mojo/application",
     "//mojo/common:tracing_impl",
-    "//mojo/services/public/cpp/view_manager",
+    "//mojo/services/view_manager/public/cpp",
   ]
 }
 
@@ -57,8 +57,8 @@ source_set("lib") {
     "//mojo/converters/input_events",
     "//mojo/public/cpp/bindings:bindings",
     "//mojo/public/interfaces/application",
-    "//mojo/services/public/cpp/view_manager",
     "//mojo/services/public/interfaces/native_viewport",
+    "//mojo/services/view_manager/public/cpp",
     "//mojo/services/window_manager/public/interfaces",
   ]
 }
@@ -85,8 +85,8 @@ test("window_manager_unittests") {
     "//mojo/edk/system",
     "//mojo/environment:chromium",
     "//mojo/public/cpp/application",
-    "//mojo/services/public/cpp/view_manager",
-    "//mojo/services/public/interfaces/view_manager",
+    "//mojo/services/view_manager/public/cpp",
+    "//mojo/services/view_manager/public/interfaces",
     "//mojo/services/window_manager/public/interfaces",
     "//shell:test_support",
     "//testing/gtest",