Use Alpha instead of TransitionAlpha for Activity Transitions.

Bug 17406204

Chrome needs to be notified when the shared element
should be hidden. The alpha setter can be overridden,
but setTransitionAlpha cannot. By setting alpha as
well as transitionAlpha, we get the immediate effect
from transitionAlpha along with enabling a trigger
for Chrome's shared element to hide.

Change-Id: I6ecb44872fd237afe89dbb36e43aa50c98693b52