Use a carrier config to get the sim call manager

Currently the default sim call manager is set using an OEM
config. We want to switch to using a carrier config.

With this CL, carriers can set the sim call manager using
the CarrierConfigManager.KEY_DEFAULT_SIM_CALL_MANAGER_STRING
config.

Once the carrier config is rolled out we can remove the OEM config.

BUG: 21499566

Change-Id: Ic0d33673b8a6424c6c20eb6686aa29dc35d4dfcd