Desktop Mode split exit reason changes.

Previously, when using the handle menu to go from split to fullscreen,
EXIT_REASON_FULLSCREEN_SHORTCUT was used. However this is reserved for
keyboard shortcuts. This CL renames the desktop exit reason to be more
broadly applicable to desktop mode, even if the transition is not
entering desktop windowing.

Bug: 323431955
Test: Manual

Change-Id: Ie28f449a065ba29e356401ff793d1d4acead389d
6 files changed