Add method for exit split screen with shell transit

Add method for exiting split screen with shell transition if shell
transitions are enabled. This is called when exitSplitScreen is called
from the external ISplitScreen interface.

This change also fixes an issue where the split pair was never broken in
recents when exit reason was EXIT_REASON_UNKNOWN.

Bug: 320736552
Test: atest StageCoordinatorTests
Test: adb shell dumpsys activity service SystemUIService WMShell \
      splitscreen exitSplitScreen <taskid>

Change-Id: I890578cb629eb8d4674d7ad7b6b0ee6bcdea1a6b