Stop calling into system server from isApkVeritySupport

Remove the use of ro.apk_verity.mode, which was for pioneering fs-verity
years ago.

Then everything can be done in the manager class.

Ignore-AOSP-First: The ongoing clean-up only happens in internal branch
Flag: EXEMPT removing deprecate_fsv_sig
Test: atest android.security.cts.FileIntegrityManagerTest FsverityTest
Bug: 277916185
Change-Id: Ib3e65fd851e0f9d5ebd10ab0d8e164402435d974