Downgrade error level when proc file is empty

Use Slog.w instead of Slog.wtf for a few empty cases when reading
proc files.

There have been quite a few issues around reporting error at Slog
.wtf level in the past. Although Slog.wtf should not crash itself,
it has indirectly caused a few crashes due to improper handling of
wtf errors e.g. arose from incorrectly mocking BatteryStatsImpl.

Test: Manual
Change-Id: Idce018c928a1bb0f3d4c139382d090be009c291d