libfiemap: Add helpers to remove images from recovery.

ImageManager can map images in recovery, but not delete them, because
/data is not mounted. libsnapshot handles this by storing extra state
files, but this is complex to manage and inconvenient for
fs_mgr_overlayfs.

Instead, this patch introduces two new calls:

 - DisableImage(), which indicates the image should not be used. This is
   implemented by adding a new DISABLED attribute to
   LpPartitionMetadata. CreateLogicalPartitions ignores this flag, and
   thus recovery/fastbootd can disable the scratch partition and
   communicate that it can be deleted. This cannot be called from binder
   since it is intended for recovery/first-stage init only.

 - RemoveDisabledImages(), which walks the images for a given folder on
   /metadata and deletes any that are disabled. This can be called from
   binder.

Note that there is no metadata version bump for this flag. It's
considered to be included in the flag list for minor version 1, and
currently is not used for the actual super partition.

Bug: 134949511
Test: adb remount, fastboot flash system
Test: fiemap_image_test
Change-Id: Iaeca2d1eddb5637dd9a20202cafd11ae60b4d0e3