Fix customized activity open/close animation backdrop color

Before, we always read the next app requested color. Now, we read the
backdrop color from next app requested animation, which is the requested
color if set, or the color in the animation XML if there is no override
color.

Bug: 241043533
Test: verify with resource color
Change-Id: I0687078a1cbcb46e71ab269059911072eb7046fd