Metrics for touch latency

Add some basic metrics for touch latency here.
Currently just measure the time from evdev to eventhub.
This will later become "hard interrupt" to eventhub, after we fix
b/119840121.

Bug: 111431676
Test: set reporting interval to 5 seconds instead of 5 minutes, then run
'statsd_testdrive 34' and touch the screen for a minute. Review the
results to ensure that times are reported as expected.
Change-Id: I6144aa0feab897a161f1ee058686dd0a790d17f1