base-bearer: avoid conncheck or stats update while disconnecting

We don't want the connection status checks to interfere with the
disconnection logic, e.g. if they're both using the same TTY for both
things.

E.g. the CGACT? from the conncheck gets in the way of the
disconnection logic:

    <debug> [1576037519.710684] Flashing data port (ttyUSB1)...
    <debug> [1576037519.710740] (ttyUSB1): port attributes not fully set
    <debug> [1576037520.287636] (ttyUSB1) device open count is 3 (open)
    <debug> [1576037520.287804] (ttyUSB1): --> 'AT+CGACT?<CR>'
    <debug> [1576037520.711067] (ttyUSB1) device open count is 2 (close)
    <debug> [1576037520.711127] (ttyUSB1): running init sequence...
    <debug> [1576037520.711231] PDP disconnection already sent
    <debug> [1576037520.711263] Disconnected bearer '/org/freedesktop/ModemManager1/Bearer/0'

Instead, fully skip all conncheck and stat updates as long as the
modem is not connected. The actual conncheck and stat update timeouts
will be removed once completely disconnected, as it was before.

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