Enable rules to allow shell to getattr on all block files
for checking modes under /dev/block.

Exempt shell from any neverallows on blk_file and limit them
to only getattr.

bug: 28306036
Change-Id: Ic26c0f7acfb238ff78d5d3537d51c1a70c64d196
Signed-off-by: William Roberts <william.c.roberts@intel.com>