More Box params

This CL adds borders, backgrounds and shapes support to the box, as well as rewriting padding setting.
Currently borderSize will be used as a default padding, as it assumed
as a most common need with the easy escape pass where you can set.

RelNote: Add paddings, border, shape and background param to Box

Also, samples were added.

Bug: 148147281
Test: b/148018195
Change-Id: I05027a87956b6e4233a6b8992d321633e9fdcdc9

diff --git a/ui/ui-unit/api/public_plus_experimental_current.txt b/ui/ui-unit/api/public_plus_experimental_current.txt
index 4653bc4..12bacd6 100644
--- a/ui/ui-unit/api/public_plus_experimental_current.txt
+++ b/ui/ui-unit/api/public_plus_experimental_current.txt

@@ -65,8 +65,10 @@
   public static final class Dp.Companion {
     method public androidx.ui.unit.Dp getHairline();
     method public androidx.ui.unit.Dp getInfinity();
+    method public androidx.ui.unit.Dp getUnspecified();
     property public final androidx.ui.unit.Dp Hairline;
     property public final androidx.ui.unit.Dp Infinity;
+    property public final androidx.ui.unit.Dp Unspecified;
   }
 
   public final inline class DpCubed implements java.lang.Comparable<androidx.ui.unit.DpCubed> {