Use lighter background for Recents

Also, re-enable gradient on tablets
(gradient will be disabled in device-specific
overlays on devices that don't render it well)

Bug: 7381275

Author: mikejurka

packages/SystemUI/res/drawable-sw600dp/status_bar_recents_background.xml

@@ -18,8 +18,8 @@
 -->
 <shape xmlns:android="http://schemas.android.com/apk/res/android">
     <gradient name="status_bar_recents_background"
-        android:startColor="#e6000000"
-        android:endColor="#c0000000"
+        android:startColor="#E0000000"
+        android:endColor="#99000000"
         android:angle="@integer/status_bar_recents_bg_gradient_degrees"
         />
 </shape>