[SharingHub] Use correct icon colors in dark mode

Apply an app:tint to the icons in the sharing hub, so they correctly
change color when the system is in dark mode.

screenshot: http://screen/zsopm5J1Tg1.png

Bug: 1009124
Change-Id: I4fbf93959829ca570aa5c0b33837ab017e5744a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974694Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726110}

chrome/android/java/res/layout/share_sheet_item.xml

@@ -5,6 +5,7 @@
 
 <LinearLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:app="http://schemas.android.com/apk/res-auto"
     xmlns:tools="http://schemas.android.com/tools"
     android:clickable="true"
     android:textAppearance="@style/TextAppearance.BlackCaptionDefault"
@@ -20,6 +21,7 @@
         android:layout_marginTop="12dp"
         android:layout_width="24dp"
         android:scaleType="fitCenter"
+        app:tint="@color/standard_mode_tint"
         tools:ignore="ContentDescription" />
     <TextView android:id="@+id/text"
         android:layout_width="fill_parent"