Record some logcat output with crashes, ANRs, etc..

Shelling out to logcat from the system server makes me queasy,
so this is turned off by default -- it must be enabled individually
for each error type (system_app_anr, etc) via a secure settings
value (which I plan to poke into from gservices for internal use).

Even when enabled, it happens in a side thread, unless the system
server is about to die anyway (system server restart).

Change-Id: Id6d88bcd78d3625f0364a5fe9c771046601a5a14