TextView should report in an AccessibilityNodeInfo's text its hint if the View text is empty.

1. TextView was reporting only its text as the text of the emmited AccessibilityNodeInfo
   not using the hint if the text is empty which is inconsistent with the handling of
   accessibility events.

2. TextView was using the contentDescription in AccessibilityEvents if its
   text and hint are empyty. However, the content description is reported as
   a separate event property and it is incorrect to report it as the text.

bug:5129138

Change-Id: I1c9270459cb07650a0ec5181cc4a01993dde3b5c