Update some UI in PhotoPicker

- Adjust the height of the add button on the bottom bar
- Remove the gradients and set solid color to toolbar and bottom bar
  on the Preview tab in the Portrait mode.
- Update the height of the bottom bar on the preview tab

Test: video and screenshot on the bug
Test: atest PreviewSingleSelectTest
Bug: 204312505
Bug: 204743889
Change-Id: I30ca264b9a63fc8c6eefdddc058381b3b2b26956
diff --git a/res/values/dimens.xml b/res/values/dimens.xml
index 64050a6..c08e41a 100644
--- a/res/values/dimens.xml
+++ b/res/values/dimens.xml
@@ -57,8 +57,7 @@
     <dimen name="picker_chip_radius">16dp</dimen>
 
     <!-- PhotoPicker Preview -->
-    <dimen name="preview_buttons_margin_horizontal">16dp</dimen>
-    <dimen name="preview_buttons_margin_bottom">10dp</dimen>
+    <dimen name="preview_buttons_padding_horizontal">16dp</dimen>
     <dimen name="preview_deselect_padding_start">2dp</dimen>
     <dimen name="preview_viewpager_margin">20dp</dimen>
     <!-- PhotoPicker Preview text -->