Move the ndk back to android-9

Author: alexcrichton

master/master.cfg

@@ -864,7 +864,7 @@ class ConfigCommand(object):
 
         # Set up the path to the android NDK
         if testing_android:
-            s += " --arm-linux-androideabi-ndk=/android/ndk-arm-18"
+            s += " --arm-linux-androideabi-ndk=/android/ndk-arm-9"
             s += " --i686-linux-android-ndk=/android/ndk-x86"
             s += " --aarch64-linux-android-ndk=/android/ndk-aarch64"
             s += " --disable-docs"