mediaserver: remove /system/bin/toolbox exec access

In Android 5.1, mediaserver couldn't execute any file on
/system. This slightly regressed due to
8a0c25ef, which granted mediaserver
access to execute /system/bin/toolbox and /system/bin/toybox

Revoke that unneeded access and add a neverallow rule to prevent
regressions.

TODO: Remove toolbox_exec:file execute permissions from domain.te
and add it back to the specific domains that need it.

Change-Id: Ia7bc6028a9ffb723d4623d91cbe15c8c1bbb2eb9