Phenomenon:
User leave from TetherSettings after Hotspot activated
and return to it later, HotspotPreference summary will
show empty.

Rootcause:
HotspotPreference.onBindViewHolder will re-hold after
user return to TehterSettings, but summary haven't
latest record, so summary will show empty.

Solution:
Get HotspotPreference to retain the latest summary after
setSummary be called.

Change-Id: Ibc2728e47c938d74819198719087098ea9983ac2
CRs-Fixed: 1088843