Rate-limit how often we collect wtf() logs.

We've seen that aggressive Log.wtf() calls can DoS the system
process, since we fork and collect recent logcat output for each
call.  After this CL, we rate-limit to only perform this heavy
lifting 5 times during any given 10 second window of time.

We still log the wtf() message to the binary event log.

Bug: 28117774, 28052546, 27994717, 27381069
Change-Id: Ic91b23fe7ee26818083a8d40fd4850fcb9d93643