net: activity_stats: Add statistics for network transmission activity

When enabled, tracks the frequency of network transmissions
(inbound and outbound) and buckets them accordingly.
Buckets are determined by time between network activity.

Each bucket represents the number of network transmisions that were
N sec or longer apart. Where N is defined as 1 << bucket index.

This network pattern tracking is particularly useful for wireless
networks (ie: 3G) where batching network activity closely together
is more power efficient than far apart.

New file: /proc/net/stat/activity

output:

Min Bucket(sec) Count
              1 7
              2 0
              4 1
              8 0
             16 0
             32 2
             64 1
            128 0

Change-Id: I4c4cd8627b872a55f326b1715c51bc3bdd6e8d92
Signed-off-by: Mike Chan <mike@android.com>