By a popular demand I have written a few steps how I am building espeak-ng for android.
ESpeak-ng has a optional dependency on a library called libsonic.
It's not critical for its functionality but it allows speeding up synthesised voice so we can get eSpeak-ng speaking at very very fast speech rates.
On linux the build system is configured to take advantage of system wide installation of all the required libraries. This does not nicely apply to the cross compilation for Android as all the libraries have to be prebuilt for the target platform.
And since there is no other distribution channel for the libsonic library other than its git repository with its source code, there are no prebuilt libraries for android.
To workaround this I am cloning libsonic repository, linking its files to the eSpeak-ng source tree and then patching eSpeak-ng source code to include the libsonic files as if these were part of eSpeak-ng it-self.

I am running all this on arch linux, but debian, ubuntu and any other linux distro with gradle, Android-SDK 26, android-sdk-build-tools 28.0.3, recent enough version of android-ndk will work fine.
I like to install all these components via the linux distro package manager.
Espeak-ng Readme has even more details I can write here. There are also some android specific dependencies

If everything is installed here are the commands I can run from the terminal:
mkdir ~/espeak # create a working folder
cd ~/espeak # enter that folder
git clone https://github.com/espeak-ng/espeak-ng.git # clone espeak-ng git repo
git clone https://github.com/espeak-ng/sonic.git # clone libsonic repository
ln -s ../../../sonic/sonic.{c,h} espeak-ng/src/libespeak-ng/ # link sonic files to eSpeak-ng source tree
wget https://files.pvagner.tk/f/e9c6874015fd4241b6c2/?dl=1 -O espeak-ng-libsonic-android.patch # download a patch
cd espeak-ng # change directory to the espeak-ng repo
patch -p1 < ../espeak-ng-libsonic-android.patch # apply the changes from the patch
./autogen.sh # prepare the configure script
./configure --prefix=/usr --with-gradle=gradle --with-extdict-ru --with-extdict-zh --with-extdict-zhy # run the configure script
make apk-debug # build the actual apk

The resulting app is placed in the path android/build/outputs/apk/debug/espeak-debug.apk
You can install this on your device and enjoy fast rates up to 800 WPM.