Implement SubscriptionController#setPreferredData.

Implement setPreferredData which sends the command to PhoneSwitcher.
Inside PhoneSwitcher, mPreferredDataSubscription should overwrite
mDefaultDataSubscription to decide which phone to activate for
default connections.

Bug: 115550764
Test: unittest
Change-Id: I3d2b4f13f78e9936fdd4a295e399dd8e071841b5
Merged-In: I3d2b4f13f78e9936fdd4a295e399dd8e071841b5