fix: repair the Android build (#905)

Rather than hardcoding the NDK version inside a script, encode it
as a file in the filesystem, which is easier to share.

Make sure we use the desired NDK by setting environment variables.

Use `-androidabi 21`, which:

1. is what rclone did: 8390ba4ca9

2. is the minimum ABI used by probe-android: 994651be52/app/build.gradle (L10)

Part of https://github.com/ooni/probe/issues/2130

.github/workflows/android.yml

@@ -5,6 +5,7 @@ on:
     branches:
       - "release/**"
       - "fullbuild"
+      - "androidbuild"
     tags:
       - "v*"