New VDM API for activity launch control.

- VD policy for defining the default activity launch behavior.
- Add/remove APIs for defining components that are exempt from the
  default policy.

s/Exceptions/Exemptions/g for consistency.

Bug: 278829484
Bug: 298418443
Bug: 298420101
Test: See CTS CL in topic
Change-Id: I921e33fa0029351951a85349cbb05260f4fc56b3