Android: Fix talkback silence when changing slider value

Previously, Talkback would not report the new value of a slider
when adjusted; in order to hear Talkback report the new value, user
had to un-focus and re-focus the slider. Changed the AccessibilityEvent
that is fired when detecting a slider value-change from
TYPE_VIEW_SCROLLED to TYPE_VIEW_SELECTED, which triggers Talkback to
report value of the focused item. This mimics Talkback behavior
of native Android sliders.

Bug: 836554
Change-Id: I4bb3bfafdb4d077f80e1ba4f3c9eecf8a7536497
Reviewed-on: https://chromium-review.googlesource.com/c/1308978Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604021}

content/public/android/java/src/org/chromium/content/browser/accessibility/WebContentsAccessibilityImpl.java

@@ -946,7 +946,9 @@ public class WebContentsAccessibilityImpl extends AccessibilityNodeProvider
 
     @CalledByNative
     private void handleSliderChanged(int id) {
-        sendAccessibilityEvent(id, AccessibilityEvent.TYPE_VIEW_SCROLLED);
+        // Sending a TYPE_VIEW_SELECTED event triggers talkback to report new
+        // slider value
+        sendAccessibilityEvent(id, AccessibilityEvent.TYPE_VIEW_SELECTED);
     }
 
     @CalledByNative