Added support for a second drop target view in DropTargetManager

This change introduces support for managing a second drop target view
within the DropTargetManager. Also added tests that verify updated
behavior.

Bug: 403363673
Flag: com.android.wm.shell.enable_create_any_bubble
Test: DropTargetManagerTest
Change-Id: Ibffcd85184ce67923c9ad6c26123f48c5adc29a2