logd: throttle SELinux denials to 20/sec

Impose a limit of 20 selinux denials per second. Denials beyond
that point don't add any value, and have the potential to cause
crashes or denial of service attacks.

Do some other misc cleanup while I'm here.

Bug: 18341932

(cherry picked from commit c234a1b8)

Change-Id: Ic5c96003db82e504baf301e87be5c8c539c73c2a