travis: use Android NDK 16b (#16562)

.travis.yml

@@ -152,10 +152,10 @@ matrix:
         - export GOPATH=$HOME/go
       script:
         # Build the Android archive and upload it to Maven Central and Azure
-        - curl https://dl.google.com/android/repository/android-ndk-r15c-linux-x86_64.zip -o android-ndk-r15c.zip
-        - unzip -q android-ndk-r15c.zip && rm android-ndk-r15c.zip
-        - mv android-ndk-r15c $HOME
-        - export ANDROID_NDK=$HOME/android-ndk-r15c
+        - curl https://dl.google.com/android/repository/android-ndk-r16b-linux-x86_64.zip -o android-ndk-r16b.zip
+        - unzip -q android-ndk-r16b.zip && rm android-ndk-r16b.zip
+        - mv android-ndk-r16b $HOME
+        - export ANDROID_NDK=$HOME/android-ndk-r16b
 
         - mkdir -p $GOPATH/src/github.com/ethereum
         - ln -s `pwd` $GOPATH/src/github.com/ethereum