Use mLastReportedPictureInPictureMode to check PiP state.

Minor correction to use the state field to more accurately check
whether the previously resolved config is in Pip mode.

Test: atest ActivityRecordTests
Flag: com.android.window.flags.enable_dragging_pip_across_displays
Bug: 431280403
Change-Id: I3ea30c542e4058a615178bcd9d8f8918bdc8a06d