[DS] Data supports DSDS

Main modification:
1. Handle network requests for different subscriptions according
to the priority of the ApnContext
2. Handle data attach and detach procedures when switching the active
data subscription

Modifications in TelephonyNetworkFactory:
1. needNetworkFor; network request are dispatched to DctController for
determining the active data subscription.
2. releaseNetworkFor; DctController is notified that the network request is
going to be released and the active data subscription may be changed
after the release of network request.

DctController maintains all cellular network requests.  When a new request
is sent to DctController the active data subscription would be determined
again. If active data subscription has not changed, the reference count of
corresponding ApnContext is increased. Otherwise, data detach procedure
will be triggered on previous data subscription and a data attach procedure
is triggered on the new active data subscription.

After data is attached, all network requests for the subscription will be
executed. That means reference count of corresponding ApnContext will be
increased.

When a network request is going to be released, the rererence count of the
ApnContext will be decreased. DctController will determine active data
subscription again after the request is released and data detach and attach
procedures will be triggered appropriately.

Change-Id: Ie991096a986e4580efd453f0d20e43971e3fa9a2