apmanager: add ProcessFactory for better unittesting

Use ProcessMock instead of real process when starting hostapd and
dnsmasq during unittest.

BUG=chromium:431759
TEST=USE="asan clang" FEATURES=test emerge-$BOARD apmanager
     Start AP service using apmanager and verify client can connect
     to it.

Change-Id: Ifea32a2dd08247db9ea4306ecef5947b305a3c8d
Reviewed-on: https://chromium-review.googlesource.com/239975
Trybot-Ready: Zeping Qiu <zqiu@chromium.org>
Tested-by: Zeping Qiu <zqiu@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Zeping Qiu <zqiu@chromium.org>
diff --git a/apmanager.gyp b/apmanager.gyp
index da70d22..87255a5 100644
--- a/apmanager.gyp
+++ b/apmanager.gyp
@@ -57,6 +57,7 @@
         'dhcp_server_factory.cc',
         'file_writer.cc',
         'manager.cc',
+        'process_factory.cc',
         'service.cc',
         'shill_proxy.cc',
       ],
@@ -107,6 +108,7 @@
             'mock_dhcp_server_factory.cc',
             'mock_file_writer.cc',
             'mock_manager.cc',
+            'mock_process_factory.cc',
             'mock_service.cc',
             'service_unittest.cc',
             'testrunner.cc',