Add overlay gradient for photos grid's item

- Show the overylay gradient for check icon and badges
- Adjust the margin of the gif badge
- Add NonNull for some methods

Test: manual. screenshots on bug
Bug: 185800908
Change-Id: I9e00db2298419fd27641df3680e972807b1ec8e3
Merged-In: I9e00db2298419fd27641df3680e972807b1ec8e3
(cherry picked from commit 8d18030fe92312cad965a18e799d8f404ac94905)
diff --git a/res/values/colors.xml b/res/values/colors.xml
index 0f814f5..243bd52 100644
--- a/res/values/colors.xml
+++ b/res/values/colors.xml
@@ -28,6 +28,8 @@
     <color name="picker_date_header_text_color">#3C4043</color>
     <color name="picker_toolbar_icon_color">#3C4043</color>
     <color name="picker_toolbar_chip_text_color">#5F6368</color>
+    <color name="photos_item_gradient_color">#42000000</color>
+
 
     <!-- PhotoPicker Preview -->
     <color name="preview_default_blue">#8AB4F8</color>