Enable from-text stub build for android_system_server_stubs_current

The module was enabled to build with from-text stub, but it was
accidentally configured to always build with from-source stubs with
aosp/3205867. This change re-enables from-text stub building for system
server stubs by creating a new build target
`android-non-updatable.stubs.system_server`, which is responsible for
from-text vs from-source toggling.

Test: CI
Flag: EXEMPT bug fix
Bug: 368048922
Change-Id: I6dbe222a416311a5194da60fd071334473a19384