Add split exit reasons

- These map to logging states, but can be used to distinguish
  cases to retain/remove the splits in recents

Bug: 202740477
Test: atest WMShellUnitTests
Test: adb shell dumpsys activity service SystemUIService WMShell
Change-Id: Ie028f6a2cb63beda97482e86e2c60dbf8f81e625
4 files changed