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_chromeos.cc b/image_properties_chromeos.cc
index 39ddeb3..87c32f0 100644
--- a/image_properties_chromeos.cc
+++ b/image_properties_chromeos.cc
@@ -118,6 +118,7 @@
                            constants::kOmahaDefaultProductionURL);
   // Build fingerprint not used in Chrome OS.
   result.build_fingerprint = "";
+  result.allow_arbitrary_channels = false;
 
   return result;
 }