Add android.os.flagging.PlatformAconfigPackage in preload list

PlatformAconfigPackage is the class used by the non-updatable part in
the system. This change uses the zygote optimization until the automated system picks up the class.

Test: presubmit
Bug: 386382756
Change-Id: I4ee9cac0f127c84154f41004387439a6a3bfcdc1