Refine ART service Proguard settings

Remove the extra rule for keeping JobService subclasses, as the
necessary rule for BackgroundDexOptJobService is now implicitly
generated and included from the framework manifest.

Bug: 272495195
Test: m + check `BackgroundDexOptJobService` in service-art dex
Change-Id: I2fab8e87e97d46cae0a960a94b2a5f5250dd33a7
Ignore-AOSP-First: Required internal ART services changes.