[PiP on CD] Update PipDesktopState#isPipInDesktopMode

to include rootTaskDisplayAreaOrganizer.isDisplayDesktopFirst check.
This ensures that on a Desktop-first display with an empty desk,
isPipInDesktopMode will still return true.

Bug: 416788154
Test: atest WMShellUnitTests:com.android.wm.shell.pip2.phone
Test: Manual - enter PiP on external display, close all other tasks.
Expand PiP and verify corners are rounded.
Flag: com.android.window.flags.enable_connected_displays_pip

Change-Id: Id613e9c6b84217b92d216d22564f906a24c42b0a