Migrate 'csp_enabled' to CarrierConfigManager.

This variable controls whether SIM cards are allowed to disable manual
carrier selection, so it makes more sense for it to be managed as
carrier config.

Currently the variable is only used in device overlays
for specific networks, but we are dropping the device-overlay so that
this simply applies to those networks.

Change-Id: Ia6ba6a89c94c0916c19bdf792e48a4feac87eded