Reuse widget picker's surface color in add item dialog.

In ag/28129361 add item dialog's color was changed to reference
material color token defined in launcher theme. But add item activity
inherits from Theme.Translucent.NoTitleBar for its appearance (while
launcher theme uses deviceDefault.Light), so, cannot inherit as is from
the launcher theme. Since, there is already a token specific for widget
picker surface color imported via widgetTheme, using it so it stays
in sync with main picker.

Bug: 354604590
Test: Manual
Flag: EXEMPT BUGFIX
Change-Id: I61ed5e65b0efd065490a54251028cde5888e4391