Don't use anim background for translucent windows

When a translucent window transitions in or out the activity behind
it does not animate. In such cases if a background color is specified
for the translucent window animation then the background will obscure
the static window behind the animating window for the duration of the
animation. This change eliminates the background color for
translucent windows.

Fixes bug 16219830.

Change-Id: I5834595afa5beae95ac2fcf8f2bad1a59271e08a