Only log dropped events every two seconds

If a device enters a state where the Sensor Event Connection's cache
is full, it would generate a log for every new event it received which
led to a large number of logs. This patch rate-limits how often the
dropped event log line is output and summarizes the number of events
that were dropped.

Bug: 120790102
Test: Builds, verified log was generated at most once every two
      seconds per SensorEventConnection

Change-Id: I61d84bc15dd72d5d36d250ce40ace200d549a6c0