Allow arbitrary channels in Android.

Add a new image property allow_arbitrary_channels which is true on
Android and false on Chrome OS.

Bug: 72259103
Test: adb shell update_engine_client --channel=foo-channel
Change-Id: I70a0980fd877ee32d6152595f8d4462bf43254a2
diff --git a/omaha_request_params.h b/omaha_request_params.h
index 73edd6f..9e41716 100644
--- a/omaha_request_params.h
+++ b/omaha_request_params.h
@@ -253,10 +253,11 @@
   }
 
  private:
-  FRIEND_TEST(OmahaRequestParamsTest, IsValidChannelTest);
   FRIEND_TEST(OmahaRequestParamsTest, ChannelIndexTest);
-  FRIEND_TEST(OmahaRequestParamsTest, ToMoreStableChannelFlagTest);
   FRIEND_TEST(OmahaRequestParamsTest, CollectECFWVersionsTest);
+  FRIEND_TEST(OmahaRequestParamsTest, IsValidChannelTest);
+  FRIEND_TEST(OmahaRequestParamsTest, SetTargetChannelInvalidTest);
+  FRIEND_TEST(OmahaRequestParamsTest, ToMoreStableChannelFlagTest);
 
   // Returns true if |channel| is a valid channel, false otherwise.
   bool IsValidChannel(const std::string& channel) const;