fix: allow to pass custom ndkVersion to the runtime

[farfromrefug]

@FestplattenSchnitzel This answer the need for this PR #1670

Instead of fixing the version i went for allowing to set it through command line.
You can then build the android runtime like this:
./gradlew -PndkVersion=20.1.5948944

@FestplattenSchnitzel why version 20.x and not 21.x ? I understand 22 might be too new but 21 should be good, right?
I just want to make sure we use a recent ndk for the runtime. I dont want any crash to appear on f-droid as we would be using an old ndk (which the dev would not know about as he builds with a newer one)

[FestplattenSchnitzel]

@farfromrefug when building locally on my machine Gradle automatically used NDK version 20.0.5594570. But if you're sure that 21.x works, we can use that one.

I dont want any crash to appear on f-droid as we would be using an old ndk (which the dev would not know about as he builds with a newer one)

I don't like crashed either, but every version built by the F-Droid test CI so far worked perfectly fine, so the chances seem to be good.

[farfromrefug]

@FestplattenSchnitzel it is not a question of me being sure it is good. As we dont define it right now it is gradle which chooses it. So it must often be quite recent.

[lambourn]

maybe related:
#1659

[farfromrefug]

@FestplattenSchnitzel will be released with the runtime 8.2