Add support to update_engine_client for -app_version and -omaha_url.

These options prevent auto-detection of these parameters.
Note that this CL makes the check_for_update DBus method obsolete from
the client's point of view.

BUG=4593
TEST=unit tests, gmerged on device and tried the client with different options.

Review URL: http://codereview.chromium.org/3048008
diff --git a/omaha_request_params.h b/omaha_request_params.h
index fb4196e..16def25 100644
--- a/omaha_request_params.h
+++ b/omaha_request_params.h
@@ -67,11 +67,13 @@
 
 class OmahaRequestDeviceParams : public OmahaRequestParams {
  public:
-  explicit OmahaRequestDeviceParams() {}
+  OmahaRequestDeviceParams() {}
 
-  // Initializes all the data in the object. Returns true on success,
-  // false otherwise.
-  bool Init();
+  // Initializes all the data in the object. Non-empty
+  // |in_app_version| or |in_update_url| prevents automatic detection
+  // of the parameter. Returns true on success, false otherwise.
+  bool Init(const std::string& in_app_version,
+            const std::string& in_update_url);
 
   // For unit-tests.
   void set_root(const std::string& root) { root_ = root; }