NumberPicker uses AccessibilityNodeProvider. When creating
AccessibilityNodeInfo, the accessibility focus boolean property should
be set. Otherwise, talkback doesn't think it is accessibility focused
and won't speak the usage hint.
Fix: b/170267520
Test: tested manully with talkback on. Unit test added.
Change-Id: Icd280bce4a394ccff2a60314518109c07808338d