Have the DataConnectionStat started from BSS

ConnectivityService is going to becoming a mainline module.
The DataConnectionStat is used to listen the telephony status
change and report to BSS. This does not really relate to
connectivity. DataConnectionStat was created in
ConnectivityService and use some hidden APIs which is not
allowed for a mainline module. Thus, move the creation and
monitor to BSS directly which is more appropriate place from
its functional perspective.

Also, move the DataConnectionStat out from module scope since
it does have too much relationship with connectivity module.

Bug: 171183530
Test: m ; verify the behavior with cellular state change
Change-Id: I0c2c1b90df7a107ee75e81fd49d39d65678204c0