update the android builder's Android version

[thestinger]

Android does have a working thread-local register / emulation for #[thread_local] but the bot does not.

Note that due to #17437 (Android now requires PIE), #17448 has to land before it can be updated.

[lahwran]

has the bot been updated? what's the status of this? how difficult would it be to update?

[tamird]

As of this writing the bots are running API 18.

[alexcrichton]

Closing, we actually ended up going back to Android API level 9 for Servo

[alexcrichton]

er, for Gecko