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_android.cc b/image_properties_android.cc
index 6e743f9..fa25683 100644
--- a/image_properties_android.cc
+++ b/image_properties_android.cc
@@ -201,6 +201,7 @@
       !system_state->prefs()->GetString(current_channel_key, &current_channel))
     current_channel = GetTargetChannel();
   result.current_channel = current_channel;
+  result.allow_arbitrary_channels = true;
 
   // Brillo only supports the official omaha URL.
   result.omaha_url = constants::kOmahaDefaultProductionURL;