Fix the size of Add button to 328dp

Added new "Add" or "Select" button for preview on long press. This
button width is fixed to 328dp.

Bug: 207753575
Test: atest PreviewSingleSelectTest#testPreview_addButtonWidth
Test: atest PreviewMultiSelectLongPressTest#testPreview_selectButtonWidth
Change-Id: Iea0976526088f375a353dc6c73e6180423533769
diff --git a/res/values/dimens.xml b/res/values/dimens.xml
index 88fbe53..ebe5ac0 100644
--- a/res/values/dimens.xml
+++ b/res/values/dimens.xml
@@ -67,6 +67,8 @@
     <dimen name="preview_deselect_padding_start">2dp</dimen>
     <dimen name="preview_viewpager_margin">20dp</dimen>
     <dimen name="preview_gif_icon_size">32dp</dimen>
+    <dimen name="preview_add_or_select_width">328dp</dimen>
+
     <!-- PhotoPicker Preview text -->
     <dimen name="preview_special_format_text_size">12sp</dimen>
     <dimen name="preview_special_format_text_margin">8dp</dimen>