libmeminfo/procrank: Ignore failures when process disappears.

procrank currently fails if a process gets killed while it is reading
the stats. This behavior is a regression from the previous version of
procrank and is often undesired.

Change procrank to silently ignore the process if it detects that it had
been killed while reading the stats. If the process is still around,
then print a warning about it and continue to read stats for other
processes in the system.

Bug: 130177765
Test: Tested by deliberately killing specific process in ProcessRecord()
      constructor

Change-Id: I701808c3226bb9b3a350ccf8e67fb29b59b0d4e0
Signed-off-by: Sandeep Patil <sspatil@google.com>