NumberPicker a11y (and a11y-inspired) fixes

- Sending scroll events for accessibility.
- Sending text update events when state changes.
- Blocking text updates during fling to reduce number of events.
- Making widget focusable by default, which keeps focus from
  moving to the rest of the UI when the text box becomes invisible.
- Managing visibility of the text box alongside IME state, so the
  IME won't decide to display the password keyboard during a fling.

Bug: 21494380
Bug: 37016501
Test: Ran NumberPicker CTS
Change-Id: I459d37d4a54c91e1cb5c7ec68fe0f012b25fb740