Android: MMC IO Latency Histograms.

This patch adds a new sysfs node (latency_hist) and reports IO
(svc time) latency histograms. Disabled by default, can be enabled
by echoing 0 into latency_hist, stats can be cleared by writing
2 into latency_hist.

Change-Id: I26c77a63af39417812e3989c836b4b5b684dfa85
Signed-off-by: Mohan Srinivasan <srmohan@google.com>
Bug: 30677035