Restrict /proc/sys/vm/mmap_rnd_bits

Label /proc/sys/vm/mmap_rnd_bits so it is only readable and writable by
init. This also tightens the neverallow restrictions for proc_security.

Bug: 33563834
Test: run cts -m CtsPermissionTestCases -t \
      android.permission.cts.FileSystemPermissionTest#testProcfsMmapRndBitsExistsAndSane

Change-Id: Ie7af39ddbf23806d4ffa35e7b19d30fec7b6d410