event/lifecycle: add an Event.String method.
event/lifecycle: add an Event.String method.

Change-Id: I5d648327cf39cd3b2cf97fa08a094744528221fe
Reviewed-on: https://go-review.googlesource.com/18400
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

golang/mobile,golang/mobile,golang/mobile,golang/mobile