Add rules for snapshotctl

snapshotctl is a shell interface for libsnapshot. After rebooting
into an updated build, on sys.boot_completed, init calls
snapshotctl to merge snapshots. In order to do that, it needs to:
  - Talk to gsid to mount and unmount COW images
  - read the current slot suffix to do checks (and avoid merging
    snapshots when it shouldn't).
  - read / write OTA metadata files to understand states of
    the snapshot
  - delete OTA metadata files once a snapshot is merged
  - collapse the snapshot device-mapper targets into a plain
    dm-linear target by re-mapping devices on device-mapper

Test: reboot after OTA, see merge completed without denials
Bug: 135752105

Change-Id: Idfe99d4004e24805d56cd0ab2479557f237c2448
diff --git a/private/file_contexts b/private/file_contexts
index c90b5c3..05ad416 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -253,6 +253,7 @@
 /system/bin/recovery-persist     u:object_r:recovery_persist_exec:s0
 /system/bin/recovery-refresh     u:object_r:recovery_refresh_exec:s0
 /system/bin/sdcard      u:object_r:sdcardd_exec:s0
+/system/bin/snapshotctl      u:object_r:snapshotctl_exec:s0
 /system/bin/dhcpcd      u:object_r:dhcp_exec:s0
 /system/bin/dhcpcd-6\.8\.2	u:object_r:dhcp_exec:s0
 /system/bin/mtpd	u:object_r:mtp_exec:s0