Rewrite #restartSystemServer

ProcessInfo#getStartTime is not reliable. My local test shows it is
incremented by one after polling a few times even if we don't restart
system server at all.

Let's use pid to detect whether system server is restarted or not.

Now #restartSystemServer fails correctly when `am restart` is not called
at all or is called without root privilege.

Bug: 160754072
Test: StagedInstallInternalTest
Change-Id: Iebcf1583f38c17fbc8f2af42bdc61ae000b39354