NDK 18 drops support for API levels below 16. Bump it to 16. NDK 18 also needs a newer version of the Android gradle plugin. Update the bind example and test build.gradle files. Change-Id: I71499c88b48c43b2d4da50e415cb0b0dcbbbed75 Reviewed-on: https://go-review.googlesource.com/136775 Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>