Don't handle click events on the emergency button in bouncer mode

This fixes a bug where the emergency call button was hidden, but not
gone.  This adds a call to setVisibility(INVISIBLE) when the alpha
is animated to 0 and restore it when bouncer mode is dismissed.

Fixes bug 7498389

Change-Id: I0898542ff7c666c8f41175a330b72d3450475739