Clarify unbuffered dispatch may fail before attached to window

Rephase the docs to to clarify that prior to Android U, the call to
`View::requestUnbufferedDispatch` will do nothing if the View is not
attached to a window.

Bug: 268105812
Test: manual
Change-Id: Ic70ee2fe5467d2471f9f95171b4feff244c6cea0
1 file changed