gn2bp: use proper target annotation

Test: //components/cronet/android:cronet
Change-Id: I72bbbec7d49c82d1581769a4968764dd3234ceda
diff --git a/tools/gn2bp/gn_utils.py b/tools/gn2bp/gn_utils.py
index 17722db..f051421 100644
--- a/tools/gn2bp/gn_utils.py
+++ b/tools/gn2bp/gn_utils.py
@@ -224,13 +224,13 @@
 
   def _get_arch(self, toolchain):
     if toolchain == '//build/toolchain/android:android_clang_x86':
-      return 'x86'
+      return 'android_x86'
     elif toolchain == '//build/toolchain/android:android_clang_x64':
-      return 'x86_64'
+      return 'android_x86_64'
     elif toolchain == '//build/toolchain/android:android_clang_arm':
-      return 'arm'
+      return 'android_arm'
     elif toolchain == '//build/toolchain/android:android_clang_arm64':
-      return 'arm64'
+      return 'android_arm64'
     else:
       return 'host'