[Motion] Split-shade transition on LS: cross-fade status bars

Before, both the keyguard status bar and the split shade header
were showing at the same time during the shade expansion.

With this change, we make sure the keyguard status bar is faded
out before the split shade header fades in.

Fixes: 226548318
Test: Unit tests + Manually
Change-Id: I5757e7d6c065ee156030e434c19a965f8e1ef2f3