Change icon mappings in MobileSignalController

The specific goal was to remove NETWORK_TYPE_LTE_CA from being used in the controller.
In order to make this smoother, the logic was refactored to use a string based
hashmap instead of an array to support the carrier aggregation variations in LTE
and 5G \ LTE.

Bug: 147465477
Test: SystemUITests
Change-Id: Ice6e795675e0c8281531c3cf52ce181a460e4bfd