PhoneBase: Add auto-rat switching on capability changes

Certain modems have a configuration where the primary data subscription
can do 3/4G while secondary ones will only do 2G.

This patchset updates preferred types for each Phone when its
capabilities change. So if a modem has a limitation as described
above and correctly reports it, the telephony stack will automatically
upgrade/downgrade the RAT accordingly.

Change-Id: I1ca4f6ef94e03666d575588b2c30c78182b37598