Implement ComputerControl Activity policy

Results in us having three possible policies with their respective flags:
- None: Everything, minus PermissionController is accessible
- Relaxed: All apps specified in the params + any application that has
  no launcher Intent are accessible.
- Strict: Only apps specified in the params are accessible.

Bug: 437849470
Flag: android.companion.virtualdevice.flags.computer_control_activity_policy_strict
Test: atest ComputerControlSessionTest
Change-Id: I7516dcd77cb9d29effd8d977ae064be306c95d30