BugreportManager API review updates.

- Expand BugreportManager class javadoc
    - Explain what "privileged" means
    - Highlight only one-BR-at-a-time behavior
- Add more detail to BugreportCallback javadocs
    - Explain standard callback invocation flow
    - More detail on when BUGREPORT_ERROR_INVALID_INPUT is used
- Add @WorkerThread to {start{,Connectivity},cancel}Bugreport

Bug: 180015324
Test: make
Change-Id: Id3400b1cdd271f47bdeaf8cdc1a0dea61aafc933