Update travis android build to use ndk 15c
Update travis android build to use ndk 15c

cleeus/tangram-es,tangrams/tangram-es,tangrams/tangram-es,tangrams/tangram-es,tangrams/tangram-es,cleeus/tangram-es,tangrams/tangram-es,cleeus/tangram-es,cleeus/tangram-es,cleeus/tangram-es,cleeus/tangram-es,tangrams/tangram-es,tangrams/tangram-es