android: Update packaging to work with latest SDK

This removes dependencies on deprecated tools "ant" and "android project"

The new method relies on JAVA_HOME and ANDROID_SDK being set.

It also requires specific versions of the Android build-tools (26.0.1) and
platforms (android-23).  See updates to CONTRIBUTING.md and our Travis-CI
config for details.

Travis-CI and documentation also roll forward to latest public NDK r14b.

scripts/travis/android_setup.sh

@@ -23,6 +23,7 @@ mv android-7.1.1/* build-tools/25.0.2/
 export ANDROID_HOME=`pwd`/tools
 export JAVA_HOME="/usr/lib/jvm/java-8-oracle"
 export ANDROID_NDK=`pwd`/android-ndk-r13b
+export ANDROID_SDK=`pwd`
 export PATH=`pwd`/android-ndk-r13b:$PATH
 export PATH=`pwd`/tools:$PATH
 export PATH=`pwd`/platform-tools:$PATH