Only stage .idsig if fs-verity is supported

Old devices launched before R and upgrading to V may not support
fs-verity. If fs-verity is not supported, don't use .idsig and don't
enable fs-verity.

The installer/store should better only install with .idsig if
FileIntegrityManager.isApkVeritySupported() returns true.

Bug: 277344944
Test: android.appsecurity.cts.PkgInstallSignatureVerificationTest
Change-Id: I78708a8b0cc829e411a3ddb64ad3be080e2df9d9