shill: refactor ChromeosDaemon

Refactor ChromeosDaemon so that it no longer inherits
from brillo::Daemon. This is necessary to allow its unit
tests to run without the extra base::ExitManager inherited
from brillo::Daemon.

Do this by introducing a new ShillDaemon class, which inherits
the logic in ChromeosDaemon, and overrides brillo::Daemon
methods.

BUG: 26653263
Change-Id: Ia256bc540d261fb183aae0100946afdcdea80fb3
TEST: shill compiles and unit tests pass on Android.
TEST: shill compiles and unit tests pass on Chrome OS.
diff --git a/shill.gyp b/shill.gyp
index 1a4f0cf..89e1884 100644
--- a/shill.gyp
+++ b/shill.gyp
@@ -526,6 +526,7 @@
         'service_property_change_notifier.cc',
         'shill_ares.cc',
         'shill_config.cc',
+        'shill_daemon.cc',
         'shill_test_config.cc',
         'socket_info.cc',
         'socket_info_reader.cc',