Defer drag indicator removal until window drop transition ends

Synchronize the removal of multi-display drag indicators with the
completion of the window drop animation for a smoother user experience.

Previously, when a window drag operation ended, the drag-move indicators
were hidden immediately. If the drop triggered a window drag transition,
the indicators would disappear before the window animation completed,
creating a visual jank.

This change adds a call to remove drag indicators inside the transition
handler to resolve the issue, this ensures the indicators disappears
smoothly as part of the window's final placement animation.

We also switch to use main thread for window drag indicator surface
creation and removal. ShellDesktopThread was used in the past, because
we had heavy operations like creating SurfaceControlViewHost. But since
we have switched to use mirrored surface, doing all these operations in
ShellDesktopThread is not needed anymore.

Flag: com.android.window.flags.enable_window_drop_smooth_transition
Bug: 398992368
Test: manual and atest
Change-Id: I6f3f54de85bd564158b1cfcaf91a1955964cf575