Cleanup string comparison in CallEndpoint class.

This CL cleans up the equals method that i sused for string comparison by the CallEndpoint class. This change updates that method to rely on the standard .equals method provided by the java.utils.Objects class.

Test: this method is widely used so PS should provide coverage of this change
Bug: 288897277
Change-Id: I5863f323f921f146e46513a0f1b29631779f76af