Introduce stem keycodes for Wear

This change adds four new stem keycodes for Android Wear. These
keycodes are intended to represent the various hardware buttons
around the watch.  There is one primary stem key that will be used
for power/settings and three generic stem keys that will be
customizable.

BUG: 21903503
Change-Id: I60f87f027fa4d4b19933d96b4110019805b30895