Enable dither for DismissView

It otherwise causes banding on devices with a small color palette.

Bug: 274938944
Test: Look for banding with maximum brightness and dark background
Test: Before https://screenshot.googleplex.com/8kHhVcps35MKqDj.png
Test: After https://screenshot.googleplex.com/Az5c9ykirCQpMXi.png
Change-Id: I62a10a8e9c7f2e8293b93f44d9f4d2fedfbc428a

libs/WindowManager/Shell/src/com/android/wm/shell/bubbles/DismissView.kt

@@ -144,6 +144,7 @@ class DismissView(context: Context) : FrameLayout(context) {
         val gd = GradientDrawable(
                 GradientDrawable.Orientation.BOTTOM_TOP,
                 intArrayOf(gradientColorWithAlpha, Color.TRANSPARENT))
+        gd.setDither(true)
         gd.setAlpha(0)
         return gd
     }