Set transientlaunch before starting transient activity

This makes all transient-launch work the same as recents
transient-launch. Without this, just starting the activity
will cause focus to transfer which violates the transient
launch contract.

This also fixes the omission of transient-hide for a
newly-launched transient activity. Since there's no
restore-below, it will just add all visible tasks to
transient-hide.

Bug: 287534499
Test: Use transient-launch for a non-recents activity and
      observe it not prematurely transferring focus.
Change-Id: I8eb153da90f8ccf9c57b5d97973a5dcaa9f50963