apparmor: switch signal mediation to use RULE_MEDIATES

Currently signal mediation is using a hard coded form of the
RULE_MEDIATES check. This hides the intended semantics, and means this
specific check won't pickup any changes or improvements made in the
RULE_MEDIATES check. Switch to using RULE_MEDIATES().

Signed-off-by: John Johansen <john.johansen@canonical.com>