libdebuggerd: fix process uptime.

I was here because we have a case where timeout(1) kills logcat, but
debuggerd alleges that the process that was killed had started less than
a second ago. I'm not sure this is the problem there, but I did notice
that far too many tombstones were claiming improbably short process
uptimes. It turns out that the code was measuring the *thread* uptime,
not the *process* uptime.

Also simplify the code a bit by switching to sysinfo(2) rather than
reading a file.

Test: manual, plus the existing unit test
Change-Id: Ie2810b1d5777ad9182be92bfb3f60795dc978b24