Fix building error for multi-line dontaudit statement

All the *.conf.dontaudit files are generated from *.conf
with the command of 'sed '/dontaudit/d' $@ > $@.dontaudit',
but this command can not be applied to multi-line dontaudit statement.

Test: Set plat_policy.conf.dontaudit as the input_file parameter of
checkpolicy tool, then selinux syntax error will occur during building.

Change-Id: I281de923d8a5f0b46256ec7de4df12a1c1d7e061