GN: Don't compile battor_agent, imagediff, bitmaptools on Android

tools/battor_agent/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//testing/test.gni")
 
+# Works only on desktop platforms.
+assert(is_win || is_linux || is_mac)
+
 executable("battor_agent") {
   sources = [
     "battor_agent_bin.cc",
   ]
   deps = [
     ":battor_agent_lib",
     "//base",
     "//device/serial",
     "//mojo/public/c/system:for_shared_library",
   ]
@@ skipping 21 matching lines @@
   deps = [
     ":battor_agent_lib",
     "//base",
     "//base/test:run_all_unittests",
     "//device/serial",
     "//device/serial:test_support",
     "//mojo/public/c/system:for_shared_library",
     "//testing/gtest",
   ]
 }