GSI vbmeta.img: set rollback_index to zero

The major purpose of vbmeta.img built on GSI targets (e.g., aosp_arm,
aosp_arm64, etc) is to disable AVB. We should also set the rollback
index to zero, to prevent the device bootloader from updating the
last seen rollback index in the tamper-evident storage.

Bug: 122583908
Test: build aosp_arm64, then `avbtool info_image --image $OUT/vbmeta.img`
Change-Id: I48a49957f8dd3169003b9507fe80e519f301d5b5