iface-modem: consolidate signal quality and access tech polling

Plugins have two ways to update signal quality and access technology
values: via unsolicited messages or via polling periodically.

Instead of keeping separate contexts for polling signal quality and
access technology values, we setup a common timeout to trigger
both. This allows us to simplify in which case the explicit update is
required, whenever one is needed to be explicitly updated, the other
one should also be.

The logic now also allows plugins to return an UNSUPPORTED error in
either load_signal_quality() and/or load_access_technologies() to tell
the interface logic that the polling of the specific item shouldn't be
performed (e.g. if the updates are expected via unsolicited messages).

If both signal quality and access technology polling is flagged as
disabled, we totally disable the polling logic internally.

The new SignalCheckContext is bound to the lifetime of the object so
that we can keep the value of the supported flags until the object is
destroyed.

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