remount: Use /data for backing scratch storage.

Currently, this is only enabled for Virtual A/B devices. When /data uses
F2FS, then proper pinning support must be enabled.

Because /data cannot be mounted in recovery, we can't delete the scratch
image backed by /data while in fastbootd. Instead, we mark it as
disabled in /metadata. The remount command now has an init script that
checks for and removes a disabled scratch partition.

Bug: 134949511
Test: adb remount on V A/B device with patched F2FS
Change-Id: Ifc8720378259654472d3822e97059b6c366f601d