Cleanups for DataCallResponse

-Simplify the nullability checking.
-Have old constructor call new one.
-Use set logic explicitly for order-independent
 comparisons in hashCode()
-Add additional nullability protection
-Add defensive return of Lists

Bug: 285590499
Test: atest DataCallResponseTest
Test: manually verified calling and browsing data, checked DataNetworkController for abnormalities
Change-Id: I3916fb83ee8a0ef048925d29e6e48fb9a37fdd8a