Skip meminfo item for the gone processes

Fix race conditions that processes could get killed during meminfo
dump.

Also fixed various other issues with meminfo dump

Bug: 144503238
Test: Insert the below code before retrieving process meminfo,
      and then set a pid, run dumpsys meminfo, check the output.

    if (SystemProperties.getBoolean("sys.fake-kill." + pid, false)) {
        Process.killProcess(pid);
        Thread.sleep(1000);
    }

Change-Id: I96436800ce218309cc5fd8a21c35ad29584faf6f