proximity: Add storing LinkLossAlertLevel

This patch stores the Reporter LinkLossAlertLevel, allowing configurable
per device Link Loss Alert Level.
proximity: Add reading stored proximity LinkLossAlertLevel

This patch loads the stored LinkLossAlertLevel when the device is
probed.
proximity: Add hash table to track LinkLossAlertLevel changes

This patch adds a GHashTable to track per device LinkLossAlertLevel.
proximity: Add reading Alert Level for Link Loss

This patch adds the implementation of the read callback for the
<<Alert Level>> characteristic.
proximity: Change the default alert levels to high

High alert level is more convenient value for test scenarios.
proximity: Remove legacy reporter implementation

This patch removes the legacy Proximity Reporter implementation which
uses the old attribute server.
proximity: Avoid invalid read if device is not probed

This patch avoids invalid memory access if LinkLossAlertLevel is not
found for a given device. This scenario can happen if the device was not
previously probed.
doc: Fix LinkLossAlertLevel description

PropertyChanged for the LinkLossAlertLevel signal is only emitted
when the link is lost.
WIP