[PiP on CD] Only allow 1 PiP across multiple displays.

When a PiP transition is started, Core checks whether there is an
existing PiP and remove it when necessary in
RootWindowContainer#moveActivityToPinnedRootTaskInner.

Previously, the code only checks for an existing PiP in the current
TaskDisplayArea, but for PiP in Connected Displays, we want to check
across displays to ensure we only have 1 PiP. This CL calls
removeRootTasksInWindowingMode(WINDOWING_MODE_PINNED) directly if the
Pip2 flag is enabled.

Bug: 397769346
Test: atest PipDesktopStateTest
Test: Manual - start PiP on first display, then start PiP on second
display. Verify first PiP is removed, and second PiP is WAI
Test: Manual - start PiP, then back navigation on second app to start
PiP on the same display. Verify first PiP is removed, and second PiP is
WAI
Flag: com.android.window.flags.enable_connected_displays_pip

Change-Id: I348c632c0e86447eb4628dfee15efa21dd4b5a12