Implement auto-enter PiP2 in gesture nav [2/N]

Implement auto-enter PiP in PiP2 while in gesture
navigation mode.

Allow Launcher to register its PinnedStackListener
with ...pip2.phone.PipController.

Bug: 325481148
Test: manually swipe up to enter PiP in auto-enter
Change-Id: I06e3686219768606f464ea1bb7268fdbe0f133e3