cros: move PowerPrefs to ash

Changes:
- Move *Power* prefs to ash/public/cpp/ash_pref_names.h
- Move *PowerPrefs* to ash/system/power/power_prefs.h
- Change DevicePowerHandlerTest to browser test.

This CL will support updating power prefs based on screen brightness
change.

[email protected]

Bug: 812504
Test: tested on device, looks fine. Also migrated test coverage.
Change-Id: Id667c0c2c930287a7e8260ba22bb582b7e1a8b58
Reviewed-on: https://chromium-review.googlesource.com/986889
Commit-Queue: Qiang Xu <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: Xiyuan Xia <[email protected]>
Reviewed-by: Dan Erat <[email protected]>
Cr-Commit-Position: refs/heads/master@{#549765}
diff --git a/ash/shell_test_api.cc b/ash/shell_test_api.cc
index 5579671..1a1687f 100644
--- a/ash/shell_test_api.cc
+++ b/ash/shell_test_api.cc
@@ -48,6 +48,10 @@
   return shell_->drag_drop_controller_.get();
 }
 
+PowerPrefs* ShellTestApi::power_prefs() {
+  return shell_->power_prefs_.get();
+}
+
 void ShellTestApi::OnLocalStatePrefServiceInitialized(
     std::unique_ptr<PrefService> pref_service) {
   shell_->OnLocalStatePrefServiceInitialized(std::move(pref_service));