Build OpenFst 1.6.9 using ndk-build

• Please add $HOME/Library/Android/sdk/ndk-bundle/ in /etc/paths

Reference https://android.googlesource.com/platform/external/openfst/+/android-4.3_r1.1/Android.mk and add some modifictions.

If you want to build this openfst, just run build_for_android.sh