Remove fall through for KEYCODE_VOICE_ASSIST

Removing unintentional fallthrough for the case
KEYCODE_VOICE_ASSIST.

The code works today since KEYCODE_VOICE_ASSIST is the
last case in the switch statement, however it is bad
practice. If somone adds another case statement the
code will break.

Change-Id: Iee6234807bbe176bd94e2584de288105d6c6a7cb