Move ndkVersion setting into a shared .gradle file

[dlam]

Playground builds were overriding the installed ndk version due to agp default and lack of ndkVersion declaration in settings.gradle.

Test: ./gradlew datastore:datastore-core:tasks Change-Id: Id6d8d9cff46ba3aa3e929e4a7df4912672a12f3a

[dlam]

I tried for a bit, but couldn't figure out how to declare it in toml. I would also prefer to share the plugin version and apply it from a common place, such as from playground plugin.