Make bubble scrim be dark always

When the scrim is lighter & the pointer triangle is also
white, it's very hard to see and will not pass GAR.

Bug: 186441337
Test: atest ScrimControllerTest
Test: manual - expand bubble in dark & light theme on top of launcher
               and on top of some other app
             => notice scrim is always dark & pointer triangle is
                slightly more visible
Change-Id: I5e5c191bb813dc272284daf11e76c02ba17dc375

packages/SystemUI/src/com/android/systemui/statusbar/phone/ScrimState.java

@@ -264,7 +264,7 @@ public enum ScrimState {
         public void prepare(ScrimState previousState) {
             mFrontTint = Color.TRANSPARENT;
             mBehindTint = Color.TRANSPARENT;
-            mBubbleTint = Color.TRANSPARENT;
+            mBubbleTint = Color.BLACK;
 
             mFrontAlpha = 0f;
             mBehindAlpha = mDefaultScrimAlpha;

packages/SystemUI/tests/src/com/android/systemui/statusbar/phone/ScrimControllerTest.java

@@ -553,7 +553,7 @@ public class ScrimControllerTest extends SysuiTestCase {
         assertScrimTinted(Map.of(
                 mScrimInFront, false,
                 mScrimBehind, false,
-                mScrimForBubble, false
+                mScrimForBubble, true
         ));
 
         // Front scrim should be transparent