Allow init to set up dm-verity

Allow init to

 1. Access device mapper to set up dm-verity devices

    avc:  denied  { write } for  pid=156 comm="init" name="device-mapper" dev="tmpfs" ino=6229 scontext=u:r:init:s0 tcontext=u:object_r:dm_device:s0 tclass=chr_file permissive=0

 2. Access the metadata partition to load and store dm-verity state

    avc:  denied  { write } for  pid=1 comm="init" name="mmcblk0p25" dev="tmpfs" ino=6408 scontext=u:r:init:s0 tcontext=u:object_r:metadata_block_device:s0 tclass=blk_file permissive=0

 3. Read /sys/fs/pstore/console-ramoops to detect restarts triggered
    by dm-verity

    avc:  denied  { getattr } for  pid=1 comm="init" path="/sys/fs/pstore/console-ramoops" dev="pstore" ino=9911 scontext=u:r:init:s0 tcontext=u:object_r:pstorefs:s0 tclass=file permissive=0

These can be reproduced using the following steps:

 1. Add fs_mgr flag verify to the system partition in fstab

 2. Add a device specific init.rc handler for the init action that
    calls the built-in command verity_load_state.

Change-Id: Id8790ae4b204ca66e671eefd3820d649f1d1e7ba