Rename variables in PhoneSwitcher.

Rename variables to be more clear:
Default data sub -> non opportunistic data sub.
Preferred data sub -> opportunistic data sub.
DDS phone / sub -> preferred phone / preferred sub.

Bug: 123598154
Test: existing unittest
Change-Id: I96a138330db2b263a2f49507f40615802e752aca
Merged-In: I96a138330db2b263a2f49507f40615802e752aca