Fix wifi tile animating full-dark when slashing/unslashing

WifiTile now owns a specific tile view that will animate the tint/alpha
of the slash but not the underlying drawable. Becasue we always animate
from the disconnected icon <-> disabled icon, this fixes the problem
where the disabled icon will start off as full-dark and appear to flash.

Fixes: 63534380
Test: turn wifi on/off; runtest -x
tests/src/com/android/systemui/qs/AlphaControlledSignalTileViewTest.java

Change-Id: I824572fdf00e122b7c17104b727171d81ff104c5