Check if RssAnon and swap were read

Keep memory snapshots even if anon RSS and swap are both 0 as long as
the values were read correctly. These should happen rarely and statsd
can be configured to ignore atoms with zeros.

Bug: 145755384
Test: atest UidAtomTests#testProcessMemorySnapshot
Change-Id: I0ab13be008c48113f80269af4f815d807096b938