PiP: Attach PiP Menu at showMenu() if it's not yet attached.

Since we attach menu on #onActivityPinned callback, it's possible that
this callback happens before the PiP leash is ready. If that's the case,
we will create the menu, but wait until the first call to showMenu() to
attach it properly.

Bug: 167219812
Test: None
Change-Id: Ieff331a953e98365d0c72ebc6685a8be475b5194