Move MainThreadInitializedObject to Launcher3 Go.

Move SandboxContext to a separate class.
Remove all MainThreadInitializedObject references in Launcher3 and tests.
Remove ObjectSandbox interface and make RoboApplication extend
SandboxContext

Bug: 361850561
Test: Presubmit
Flag: EXEMPT dagger
Change-Id: I1a8af5e6ae59df44d8ad6732b4d6e949ac402d6c