Deprecate .fsv_sig

.fsv_sig is no longer used by anyone in the GMS ecosystem.

Guarde the deprecation with a flag. Also adding a host variant of flag
to be used for testing purposes.

Bug: 277916185
Test: adb shell device_config put hardware_backed_security \
              com.android.server.security.deprecate_fsv_sig false # and true
Test: atest CtsPackageManagerTestCases:android.content.pm.cts.ChecksumsTest
Test: atest CtsAppSecurityHostTestCases:android.appsecurity.cts.ApkVerityInstallTest
Change-Id: I2708e59f3ec27fa32ef9cb4e2233364bb65f7c17