Replace incomplete path-based check with statx

The current check is incomplete as it doesn't capture /system and
others. Switch to hasFsverity (which calls statx) to directly query
the filesystem the fs-verity-ness. This only needs the commom getattr
SELinux privileged, so no sepolicy change is needed.

Bug: 279597861
Bug: 258093107
Test: see 258093107#30
Change-Id: I3140b770b57c4fedc8f077adddda5ec2a50c4ee6