Intercept activity start requests in client organizer

This allows applying split rules to activity start requests for
different processes and covers APIs like 'startActivityAsUser',
not requiring app developers to use dedicated APIs to start to side
instead.

When the client organizer observes a new activity being started, it
creates a new TaskFragment first and modifies the activity start
options to target that container.

Bug: 194140227
Bug: 190433398
Test: Manual, using reference implementation and sample app.
Change-Id: Ice3de8ec725d327266ec38052129b4158608855e