Add missing GuardedBy annotation

StatsPullAtomService.readProcStatsHighWaterMark touches |mBaseDir|,
but it doens't have the GuardedBy annotation.
Adding it should be safe because the method is called only by
StatsPullAtomService.getStatsFromProcessStatsService() which already
has the annotation.

Bug: 278630339
Test: m
Change-Id: I748a20460e899785d15233344dcd3e10bb830879