Make enter reveal animation be zorder top.

This fixes the bug where launcher icons become temporarily visible when
home button is pressed while other activity is starting. By having the
starting activity's animation be zoder top, it will continue to be drawn
on top of the launcher until it animates away.

Bug: 22809202
Change-Id: If5e3c09b7a5df4537c355f94e986766f77ad4943