Ensure that revealAmount = 0 doesn't leave an oval on the right side of the screen.

This is done by animating the bounds to a height of 0, and fixing a bug where if the height was 0, we wouldn't draw anything (we should draw the end color in that case). This also fixes a bug where the scrim was not properly initialized.

Bug: 187072550
Test: manual
Change-Id: I3b1224d196cff6045624a527f488f394929f7423