net: openthread: align otPlatSettingsSet with new interface contract

The new interface contract guarantees that OpenThread stack uses
otPlatSettingsSet only for aKey which has at most one value at time.
This implies the simplification for key name used by settings subsystem
and decreases the count of records written each time when the value
for specific key is updated. In result non-volatile memory can be used
more efficiently.

It relates to zephyrproject-rtos/openthread commit: ed665e9 .

We still need to make sure that old entries are being removed for the
case with DFU.

Signed-off-by: Lukasz Maciejonczyk <lukasz.maciejonczyk@nordicsemi.no>