Turn OmahaRequestPrepAction into OmahaRequestDeviceParams.

Pass the params to OmahaRequestAction's ctor. This simplifies a bit
executing as well as testing of OmahaRequestAction and testing of
OmahaRequestDeviceParams. It also allows us to initialize the params
once per update attempt and use them for all OmahaRequestActions.

BUG=560
TEST=unit tests, gmerged on device and forced an update through dev server,
inspected logs.

Review URL: http://codereview.chromium.org/2836053
diff --git a/SConstruct b/SConstruct
index df29f86..c3960bd 100644
--- a/SConstruct
+++ b/SConstruct
@@ -210,7 +210,7 @@
                    marshal.glibmarshal.c
                    omaha_hash_calculator.cc
                    omaha_request_action.cc
-                   omaha_request_prep_action.cc
+                   omaha_request_params.cc
                    omaha_response_handler_action.cc
                    postinstall_runner_action.cc
                    set_bootable_flag_action.cc
@@ -243,7 +243,7 @@
                             mock_http_fetcher.cc
                             omaha_hash_calculator_unittest.cc
                             omaha_request_action_unittest.cc
-                            omaha_request_prep_action_unittest.cc
+                            omaha_request_params_unittest.cc
                             omaha_response_handler_action_unittest.cc
                             postinstall_runner_action_unittest.cc
                             set_bootable_flag_action_unittest.cc