fix: avif_android jni build.gradle config ndkVersion

[fdintino]

Setting ANDROID_NDK_HOME no longer has any effect in gradle. When you run ./gradlew --info the log outputs:

C/C++: Not considering ANDROID_NDK_HOME because support was removed after deprecation period.
C/C++: Because no explicit NDK was requested, the default version [23.1.7779620] for this Android Gradle Plugin will be used
C/C++: android.ndkVersion from module build.gradle is [not set]
C/C++: android.ndkPath from module build.gradle is not set
C/C++: ndk.dir in local.properties is not set

This sets android.ndkVersion so that it matches the NDK version used in the github actions workflows.

[vigneshvg]

Thank you for the PR.