Add PowerManagerInterface.

Hide all dbus stuff under the new interface, so that we can implement
a real power_manager for android in the future if needed.

Test: mma
Bug: 28800946

Change-Id: I26c883f4d0bc71f0410dfe4422b22bdd9df70575
diff --git a/update_engine.gyp b/update_engine.gyp
index d437003..d2f5416 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -56,7 +56,6 @@
       'USE_LIBCROS=<(USE_libcros)',
       'USE_MTD=<(USE_mtd)',
       'USE_OMAHA=1',
-      'USE_POWER_MANAGEMENT=<(USE_power_management)',
       'USE_WEAVE=<(USE_buffet)',
     ],
     'include_dirs': [
@@ -267,6 +266,7 @@
         'omaha_utils.cc',
         'p2p_manager.cc',
         'payload_state.cc',
+        'power_manager_chromeos.cc',
         'proxy_resolver.cc',
         'real_system_state.cc',
         'shill_proxy.cc',