aura: rename WindowTreeClientPrivate to WindowTreeClientTestApi

This better indicates it is only for tests.

BUG=none
TEST=none

Change-Id: Ife73cabedb8f7b653ad3f4102a143f1248aa508b
Reviewed-on: https://chromium-review.googlesource.com/c/1313052
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604788}

• ash/ws/ash_window_manager_unittest.cc[diff]
• ui/aura/BUILD.gn[diff]
• ui/aura/mus/embed_root.h[diff]
• ui/aura/mus/window_port_mus.h[diff]
• ui/aura/mus/window_tree_client.h[diff]
• ui/aura/mus/window_tree_client_unittest.cc[diff]
• ui/aura/test/aura_test_helper.cc[diff]
• ui/aura/test/event_generator_delegate_aura.cc[diff]
• ui/aura/test/mus/change_completion_waiter.cc[diff]
• ui/aura/test/mus/test_window_tree_client_setup.cc[diff]
• ui/aura/test/mus/window_tree_client_test_api.cc[Renamed from ui/aura/test/mus/window_tree_client_private.cc - diff]
• ui/aura/test/mus/window_tree_client_test_api.h[Renamed from ui/aura/test/mus/window_tree_client_private.h - diff]
• ui/aura/test/test_screen.cc[diff]
• ui/views/mus/desktop_window_tree_host_mus_unittest.cc[diff]
• ui/wm/test/wm_test_helper.cc[diff]