Inject long press documentation

Clarify usage of Activity.onKeyLongPress(..). It is not obvious at
all that onKeyDown should be used in order for OnKeyLongPress to work.
Direct readers to the other api to make sure that this nuance is not
missed.

Bug: 21451760
Test: atest KeyEventInjectionTest
Change-Id: Ic73f6fe06b2108a3967ac590d22acfb283a700e1