Mark fs-verity support for /metadata if first_api_level >= R

fs-verity is required for new devices launched with R.

This allows files stored on /metadata to be protected by fsverity.

Bug: 199914227
Test: mini-keyctl padd asymmetric fsv-sepolicy .fs-verity \
< /system/etc/security/com.android.sepolicy.cert.der
cp /apex/com.android.sepolicy.apex/app/SEPolicy-33/SEPolicy-33.apk \
/metadata/sepolicy/
fsverity enable /metadata/sepolicy/SEPolicy-33.apk \
--signature=/apex/com.android.sepolicy.apex/etc/SEPolicy-33.apk.fsv_sig

Change-Id: I44434e3d026f1dbe6e261c365b3c70d3556a80b1