Move 2 new FileIntegrityManager APIs to @SystemApi

Due to system health concerns of potential system wide I/O impact by 3P
apps using this API, move it from public to @SystemApi until the
underlying verity performance problem is fixed.

setupFsverity is now guarded by SETUP_FSVERITY permission, which is
signature|privileged. getFsVerityDigest is not guarded because it simply
calls a (read-only) ioctl in the app process.

Bug: 285185747
Bug: 296467543
Test: m
Test: atest android.security.cts.FileIntegrityManagerTest FsVerityTest
Test: atest android.permissionpolicy.cts.PermissionPolicyTest#platformPermissionPolicyIsUnaltered
Change-Id: Iff6141f01c24a5a8f03339fc764316c51339e686