DropBox logging of app & system server crashes.

The crashes are also reported to the event log (and of course the
main logcat, like they always have been).  Ordinary Log.e(t,m,e) isn't dropboxed
but there's a new Log.wtf() which always is.  (Still @pending in this change.)

Add a hook to IPowerManager to crash the system server on demand
(only for apps with REBOOT permission, since it's basically a restart).
This is not exposed in PowerManager, must be invoked directly -- mostly
this is there so "Bad Behavior" in dev tools can do it.