Bubble API: intent extra for bubbled activities

Apps need a better way to check if their shortcut or
pending intent is bubbled, to do this we'll add an intent
extra on the bubbled activity that they may check.

- updated doc around bubbles for how to check the extra
- applied flag on activity for intent & shortcut

Bug: 171220648
Test: atest NotificationManagerTest (see CTS CL)
Change-Id: Iba97a0058e1e4fc33c4df32a3f258ff350ad04e2