Fix back button handling in ACTV

AutoCompleteTextView.onKeyPreIme() used
getKeyDispatcherState().startTracking(event, this) to start tracking
the back button, but didn't update the state of the event
before checking event.isTracking().

Fixes http://b/issue?id=2167168
"AutoCompleteTextView back button handling is broken"

Change-Id: I2c5f1e761872b9e02d85ff6f958fabd558d041a0