Add System API to enable / disable a logical modem.

The enableModem HAL interface is already defined. Now we need to
surface it through PhoneConfigurationManager to TelephonyManager
as a SystemApi that ONS can use.

Bug: 122926141
Test: unittest
Change-Id: If83231dc12d2d3e4b6cde6dce69c5d747805f331
Merged-In: If83231dc12d2d3e4b6cde6dce69c5d747805f331