Add API for setting a content description for BigPictureStyle's picture

BigPictureStyle allows devs to attach a big picture image to their
notifications. Since images can have content descriptions, allow devs
to add content descriptions to the big picture image for a11y

Bug: b/147295675
Test: atest NotificationManagerTest; created a notification and
tested with TalkBack

Change-Id: Ibdf9d67a7d9a8c3a60b2560c3be6dde93247b7ac