Get PiP round corner radius from WMShell

Fixed also the issue that when
SystemUiProxy#setPinnedStackAnimationListener is called,
SystemUiProxy#mPiP may not have been initialized, defer the set/register
action in SystemUiProxy#setProxy if applicable.

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/dmUy8qBEMxHShFcFKB3cT3
Bug: 171721389
Test: make sure autoEnterPip has round corner support, see video
Change-Id: I38866bbc77bc2fa94f0197bb90c02e786198443e