Increment a MetricsLogger counter on back press.

For Wear, we want to know how often users press the side button, which
is mapped to a back key press. The number of times the back button is
pressed is probably a useful metric for the rest of Android.

The counter string, "key_back_down" is not yet declared in
go/tron-counters, but we should get the framework change in ASAP.

Bug: 32396371
Change-Id: Ia102318cd5ca4d077ce404944084806198c334f7