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_unittest.cc b/omaha_request_params_unittest.cc
index 57ecf24..6705c71 100644
--- a/omaha_request_params_unittest.cc
+++ b/omaha_request_params_unittest.cc
@@ -145,6 +145,7 @@
     params.set_root(tempdir_.GetPath().value());
     SetLockDown(true);
     EXPECT_TRUE(params.Init("", "", false));
+    params.image_props_.allow_arbitrary_channels = false;
     string error_message;
     EXPECT_FALSE(
         params.SetTargetChannel("dogfood-channel", true, &error_message));
@@ -167,6 +168,8 @@
   EXPECT_FALSE(params_.IsValidChannel("dogfood-channel"));
   EXPECT_FALSE(params_.IsValidChannel("some-channel"));
   EXPECT_FALSE(params_.IsValidChannel(""));
+  params_.image_props_.allow_arbitrary_channels = true;
+  EXPECT_TRUE(params_.IsValidChannel("some-channel"));
 }
 
 TEST_F(OmahaRequestParamsTest, SetTargetChannelWorks) {