[PiP on Desktop] Fix rounded corners when expanding PiP

Set applyCornerRadius to true if PiP is expanding to Desktop Mode so
that the freeform window corners are rounded appropriately.

Bug: 397765675
Test: atest PipExpandAnimatorTest PipExpandHandlerTest
Flag: com.android.window.flags.enable_desktop_windowing_pip
Change-Id: Ic56bf22752c825f5068148a8c6f0c5d4ca13b2ee