Fix for bug where we don't use default grid on comet, and migrate normally if not in a B&R case

-The grid migration where we just copy the grid and move everything one row up should only occur in a B&R scenario, so now we add that restriction
-We should default to 4x5 in comet if migrating from a 4x4 grid if the device we're migrating from does not have any other grids saved
-if we have other grids saved, then we'll try using the saved grid if possible. If not possible, we use the default grid for the new device

Bug: 360462379
Test: GridSizeMigrationUtilTest
Flag: EXEMPT bugfix
Change-Id: Ia905081046431c08dc058bd61b2b4ab42dee0506

src/com/android/launcher3/InvariantDeviceProfile.java

@@ -354,7 +354,7 @@ public class InvariantDeviceProfile implements SafeCloseable {
      */
     @Deprecated
     public void reset(Context context) {
-        initGrid(context, getCurrentGridName(context));
+        initGrid(context, getDefaultGridName(context));
     }
 
     @VisibleForTesting