Add support for persisting wallpaper selection.

app/src/main/proto/Database.proto

@@ -90,4 +90,26 @@ message DeviceLastResetTime {
     }
 
     repeated Pair resetTime = 1;
+}
+
+message Wallpaper {
+    message SingleColor {
+        int32 color = 1;
+    }
+    message LinearGradient {
+                 float rotation  = 1;
+        repeated int32 colors    = 2;
+        repeated float positions = 3;
+    }
+    message File {
+        string uri = 1;
+    }
+
+    oneof wallpaper {
+        SingleColor    singleColor    = 1;
+        LinearGradient linearGradient = 2;
+        File           file           = 3;
+    }
+
+    float dimLevelInDarkMode = 4;
 }