breakManager: Clarify some nextBreakType variables

breakManager: Clarify some nextBreakType variables

This doesn’t introduce any functional changes, as
`this._manager.currentBreakType` is set to the value returned by
`this._manager.getNextBreakDue()` when entering `BreakState.BREAK_DUE`.

However, it should make the code a little clearer as now the code refers
to the ’next’ break type rather than the ‘current’ one in the context of
an upcoming break.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3687>