PiP: Ensure PiP menu layout updates are on Shell main thread.

Only the thread which created the menu view (shell main thread) can
update its view layout, so ensuring that it is executed in that sense.

Also moved setup work in PipController to Shell Main thread. Ideally
they should be in ShellInit, but that will need to wait until
tv.PipController can unify with PipController.

Bug: 182418859
Test: Enter PIP
Change-Id: Ib88fefb9320ab441715c6ef3d5cf4f80dc9f022f