Allow Workspace Scrim to be colored per state

Make it possible to set the color of workspace scrim to a different color
per state. Motivated by making Overview Scrim and All Apps scrims different
colors.

Bug: 186253733
Test: Local build and flash
Change-Id: Id7c38ce3c9173308eedfcb7592ececa7bd6bf220
diff --git a/res/layout/launcher.xml b/res/layout/launcher.xml
index f34e685..039d8d3 100644
--- a/res/layout/launcher.xml
+++ b/res/layout/launcher.xml
@@ -66,8 +66,7 @@
             android:layout_width="match_parent"
             android:layout_height="match_parent"
             android:id="@+id/scrim_view"
-            android:background="?attr/allAppsScrimColor"
-            android:alpha="0" />
+            android:background="@android:color/transparent" />
 
         <include
             android:id="@+id/apps_view"