Lower error if diskstats cache file not found

/data/system/diskstats_cache.json is populated once a day and may not
exist right after a factory wipe. If the file does not exist, we catch
the exception and print it to logcat. This CL changes the print call
from a Slog.e to a Slog.w to reflect the fact that the situation is
undesirable, but properly handled.

Test: m
Bug: 157947470
Change-Id: Ie04a2768670ec74635fc9e050a466eaad9c6a9e9