Fade scrim in unlock animation.

This also introduces a startTime which gets sent from window manager
to SystemUI, which tells when the animation should start, to allow
for a more synchronized animation with fading out the scrim and
fading in the activity behind.

Bug: 15163546
Change-Id: I16212b1ef9eb76f1f98734da1d14fc5b7e626937
16 files changed