Set ACCESSIBILITY_LIVE_REGION_POLITE for ProgressStyle progress bar.

This bit has the following effects:
  1. When the focus moves to the notification, the talkback announced
     message includes the the progress bar description (percentage, or
     in progress).
  2. When the focus is on the notification, change in the progress bar
     triggers a talkback announcement.

Flag: android.app.api_rich_ongoing
Fix: 389986049
Test: Test talkback for notifications posted via ProgressStyle test app.
Test: Test talkback for the notification from the Android easter egg.
Change-Id: I27b519918580d0e19a92cda4015476604cfcd624