Change unit of I/O statistics to MiB

I/O in bytes are too large and it may cause overflow. Moreover, data with
large numbers are grouped in the same bucket of the histogram. This adds
difficulty to the analysis. Changing unit of I/O to MiB so that we can
have a better data distribution.

Bug: 28658632
Change-Id: Id9913d71e62b36ce5d5d2e57676953f4dbd0c7c9