API for app launch on computer control sessions

Takes only a package name, which needs to have been declared during
the creation of the session.

The app launch is done server-side on behalf of the agent app.

Bug: 437849228
Flag: android.companion.virtualdevice.flags.computer_control_access
Test: atest & presubmit & manual
Change-Id: I27e1531526df768946e78950ed6622ca8c47261a