[PiP on Desktop] Remove display windowing check in isPipInDesktopMode.

We no longer need to check the windowing mode of the display in
PipDesktopState#isPipInDesktopMode. The isAnyDeskActive check is
sufficient to indicate whether the PiP window is in an active Desktop
session.

Bug: None
Test: atest PipDesktopStateTest
Test: Manually check that PiP window expands in freeform while in
Desktop session
Flag: com.android.window.flags.enable_desktop_windowing_pip

Change-Id: Ia7ee25f051616c8161d52c999f0f4a8f894731e9