Add cross-fade animation to launcher for dark/light theme changes.

Test: manual

Fixing b/167570781. The launcher was not cross-fading it's original appearance with its new appearance on theme changes. It would only reload. Added a cross-fade animation.

Demo: https://drive.google.com/file/d/1adRIJIMFw4WGdfSqZbS0caR2LH3j_yH-/view?usp=sharing

Change-Id: I69162d9c49565e007cb33402389abf7d20806dd7