setAccessibilityFocused for NumberPicker

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