Make KeyguardMultiUserAvatar fully themeable

Create the animation color for the username text based on the final
color.

Change-Id: I4b1beb70160e178264c32f3b33c6c0c7c51f70c5
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>
Merge commit 'f271f533feb6f3aad9bfe44224ff95cd76b89d0c' into android-4.4