ACTV: getWindowVisibility() instead of private attach count

When debugging a problem with the search dialog drop-down
sometimes not showing up, I saw that the mAttachCount variable
in AutoCompleteTextView could get a negative value because
onDetachedFromWindow() was called multiple times. This lead to
the drop-down not being displayed on filtering.

Instead of the private attach count in ACTV, this change
uses View.getWindowVisibility().