SyncSM08: handle IpServer requestEnableTethering callback in mainSM

This can make it easier to adopt SM shim replacement in follow up change,
as both SyncSM and AsyncSM will process the same message without any
difference.

This change also fix a problem that bring up downstream before it's
IpServer completely disabled.

Previous:
CMD_NOTIFY_PREFIX_CONFLICT -> requestEnableTethering(false) ->
requestEnableTethering(true) -> IpServer stopped ->
reportTetherStateChanged(downstream inactive) -> IpServer started
-> reportTetherStateChanged(downstream active)
New:
CMD_NOTIFY_PREFIX_CONFLICT -> requestEnableTethering(false) ->
IpServer stopped -> reportTetherStateChanged(downstream inactive)
-> requestEnableTethering(true) -> IpServer started
-> reportTetherStateChanged(downstream active)

Test: atest MtsTetheringTestLatestSdk
Change-Id: Ibe22b300c56125359f4fa452bd01a0a2381fda23