Bug 1457669 - Support Android system headers for standalone SpiderMonkey builds. r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D29698

--HG--
extra : moz-landing-system : lando

config/system-headers.mozbuild

@@ -1048,7 +1048,7 @@ system_headers = [
     'zmouse.h',
 ]
 
-if CONFIG['MOZ_WIDGET_TOOLKIT'] == 'android':
+if CONFIG['OS_TARGET'] == 'Android':
     system_headers += [
         'android/api-level.h',
         'android/ashmem.h',