Fixing black background when going back from assistant.

- When returning to a non-wallpaper task from the assistant, the
  opaque animation background for the task-close animation was
  drawing behind the assistant stack and occluding the fullscreen
  task. The original change in ag/99642 no longer applies since
  our task switcher has changed and we can remove this opaque
  background.

Bug: 63072954
Test: Open fullscreen task, open assistant, hit back. Also ensure
      that launching and closing new tasks don't result in additional
      tasks being visible.

Change-Id: I3347a552b15d3cd5ed51f283b99e4caa5eb298a0

core/res/res/anim/task_close_enter.xml

@@ -18,7 +18,7 @@
 -->
 
 <set xmlns:android="http://schemas.android.com/apk/res/android"
-        android:background="#ff000000" android:shareInterpolator="false" android:zAdjustment="normal">
+        android:shareInterpolator="false" android:zAdjustment="normal">
 
     <alpha android:fromAlpha="0.6" android:toAlpha="1.0"
             android:fillEnabled="true" android:fillBefore="true" android:fillAfter="true"

core/res/res/anim/task_close_exit.xml

@@ -18,7 +18,7 @@
 -->
 
 <set xmlns:android="http://schemas.android.com/apk/res/android"
-        android:background="#ff000000" android:shareInterpolator="false" android:zAdjustment="top">
+        android:shareInterpolator="false" android:zAdjustment="top">
 
     <alpha android:fromAlpha="1.0" android:toAlpha="0"
             android:fillEnabled="true" android:fillBefore="true" android:fillAfter="true"