Add sepolicy for installing GSIs to external storage.

To install GSIs on external storage (such as sdcards), gsid needs some
additional privileges:
 - proc_cmdline and device-tree access to call ReadDefaultFstab().
   This is ultimately used to check whether system's dm-verity has
   check_at_most_once enabled, which is disallowed with sdcards.
 - vfat read/write access to write files to the sdcard. Note that
   adopted sdcards are not supported here.
 - read access to the sdcard block device. To enable this without
   providing access to vold_block_device, a new sdcard_block_device
   label was added. Devices must apply this label appropriately to
   enable gsid access.
 - FIBMAP access for VFAT filesystems, as they do not support FIEMAP.
   This only appears to work by granting SYS_RAWIO.

Bug: 126230649
Test: adb shell su root gsi_tool install --install_dir=/mnt/media_rw/...
      works without setenforce 0

Change-Id: I88d8d83e5f61d4c0490f912f226fe1fe38cd60ab
5 files changed