Consolidate animation to show divider bar

Add a fade-in animation for showing the divider bar. And make sure it
animates in-sync with task bar by deferring the fade-in animation until
keyguard dismissed.

When quickswitch or overview switch back to a split pair, the launch
split tasks remote animation will get onAnimationCancelled due to the
app transition was handled by recent animation controller. So leaves
divider bar fade-in animation to launcher when the remote animation got
cancelled.

Fix: 229613465
Fix: 208651526
Test: atest WMShellUnitTests
Test: launch a split pair and a fullscreen app, switch from overview or
      quickswitch back and forth between split pair and the fullscreen
      app. Observed divider bar animates in in-sync with task views.
Test: verified divider bar animates in-sync with task bar after keyguard
      dismissed.
Change-Id: I2f9064a089c446711545afda8204557a708545c5