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.

This CL re-merges change I459d37d4a54c91e1cb5c7ec68fe0f012b25fb740
but includes a fix to hide the visibility of the input text field
whenever the keyboard needs to be hidden, even if the keyboard
is not being shown or the text box isn't active. That removes an
undesirable visual side-effect of the previous change. This CL
also has a more correct decision about when to send a text change
accessibility event.

Bug: 21494380
Bug: 37016501
Test: Ran NumberPicker CTS, including a new change.
Change-Id: I04c49ddc2f877cd019ee355c722c81dcd3ba0dda