Return an empty accessibility node info for a keyboard

Because a keyboard handling hover events and determining a virtual
node by itself, there is no need to supply whole virtual nodes info
for the keyboard. Just returning an empty accessibility node info
supresses annoucements of all keys.

This CL also fixes the undefined virtual id value.

Bug: 15582251
Change-Id: Ie033d21ef878d272417cf2b20f8eec1e516587f6