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/image_properties.h b/image_properties.h
index f8fe095..49fe82f 100644
--- a/image_properties.h
+++ b/image_properties.h
@@ -58,6 +58,10 @@
   // The release channel this image was obtained from.
   std::string current_channel;
 
+  // Whether we allow arbitrary channels instead of just the ones listed in
+  // kChannelsByStability.
+  bool allow_arbitrary_channels = false;
+
   // The Omaha URL this image should get updates from.
   std::string omaha_url;
 };