I tried to use Android NDK to compile TN-Grid app and was able to prepare 3 app versions: ARM 32-bit PIE for Android 5+, ARM 32-bit non-PIE for older versions and AARCH64 PIE (minimum Android version here is 5, so non-PIE version is not needed).
AARCH64 version uses hard float ABI like ARM Linux versions. However for some reason I was not able to do the same for ARM 32-bit apps, linker complained that I cannot mix softfp and hard ABIs. According to https://developer.android.com/ndk/guides/standalone_toolchain.html this is critical for ABI compatibility. However this is slower than using -mfloat-abi=hard, so these apps will be slower than their ARM Linux counterparts.
I have installed AARCH64 version on my phone and completed successfully 17 WUs. I did not try 32-bit version, unfortunately there are no WUs now.
Apps are uploaded in the same place as other apps: https://bitbucket.org/sirzooro/pc-boinc/downloads. To install it first determine version which you need. To do this, open Event Log in BOINC Client and scroll down to the bottom. First message will be like "Starting BOINC client version 7.4.53 for aarch64-android-linux-gnu". If you will see "aarch64-android-linux-gnu" there, you should use app AARCH64 app. If you will see "arm-android-linux-gnu", you need one of apps for ARM (armv7a). Check which Android version you have. If it is Android 5 or never, use PIE version; if it is older, use non-PIE.
In order to install my app on Android first I copied it from Windows to phone using Windows Explorer. Then on phone I used Total Commander app to copy both files (app_info.xml and pc) to /data/data/edu.berkeley.boinc/client/projects/gene.disi.unitn.it_test/ dir. TC displayed some popup that it was using superuser permissions, so you may have to have rooted phone (I am not sure about this). I also used TC to change access permissions for these files (644 for app_info.xml, 755 for pc) and ownership (both user and group should be BOINC). Hint: check some other file there to get UID and GID values for user and groups. Android uses tons of users and groups, so finding proper ones using numbers instead of names will be faster. After doing this I had to restart BOINC (I restarted phone to do this).
Edit: most probably TN-Grid is not on project list in Android BOINC client now. To add it, go to the project list, then press and hold hamburger button (one with 3 horizontal lines) for a while, then enter http://gene.disi.unitn.it/test/ in box which will appear. You can also use account manager like BAM to do this.