Remove blink during the freeform -> recents transition.

We achieve the desired result by prolonging the last frame of the
animation until recents tells that it drew its content. The CL also
includes cleanup that moves code that depends heavily on WindowState
fields into that class.

Bug: 24913782
Change-Id: I5ee5b18504dd4a86c24033d17eca21cd31936bca
diff --git a/tools/layoutlib/bridge/src/android/view/IWindowManagerImpl.java b/tools/layoutlib/bridge/src/android/view/IWindowManagerImpl.java
index 498be5a..7ee06f3 100644
--- a/tools/layoutlib/bridge/src/android/view/IWindowManagerImpl.java
+++ b/tools/layoutlib/bridge/src/android/view/IWindowManagerImpl.java
@@ -517,4 +517,8 @@
     @Override
     public void cancelTaskWindowTransition(int taskId) {
     }
+
+    @Override
+    public void endProlongedAnimations() {
+    }
 }