Notification Assistant API

This API allows a single assistant on the device to help the
user manage their notification stream by taking actions on
individual notifications:

  - modifying their priority up or down when they are posted
  - possibly changing if and how the notification interrupts the user
  - adding annotations under notifications

Bug: 22455414
Change-Id: Idf47972bb71c83f1dc1c9ec68a6fa92ac4fc522f