Support select/deselect in Preview

Added support for deselecting selected items in preview.

1) Supports deselecting and selecting it back
2) Add button shows the number of items selected
3) Preview doesn't completely match with mocks
4) Doesn't support previewing videos
5) Added background color as black, toolbar will still have app theme
color.

Bug: 185801129
Test: Videos attached to the bug
Change-Id: I4b0b919a09bef7dd0708efdfe2a7465b86ecf792
Merged-In: I4b0b919a09bef7dd0708efdfe2a7465b86ecf792
(cherry picked from commit 7352ddfbb3542602bced9708d2db82223d756328)
diff --git a/res/values/colors.xml b/res/values/colors.xml
index 737e54a..e593c25 100644
--- a/res/values/colors.xml
+++ b/res/values/colors.xml
@@ -19,11 +19,15 @@
     <color name="clear_cache_icon_color">#5F6368</color>
 
     <!-- PhotoPicker -->
+    <color name="picker_default_white">@android:color/white</color>
+
+    <!-- PhotoPicker photo grid -->
     <color name="picker_primary_color">#1A73E8</color>
     <color name="picker_background_color">@android:color/white</color>
     <color name="picker_highlight_color">#E8F0FE</color>
 
     <!-- PhotoPicker Preview -->
-    <color name="preview_add_button_background_color">#8AB4F8</color>
-    <color name="preview_buttons_text_color">#202124</color>
+    <color name="preview_default_blue">#8AB4F8</color>
+    <color name="preview_default_grey">#202124</color>
+    <color name="preview_default_black">@android:color/black</color>
 </resources>