[NS A31] Simplification

The condition this is testing for cannot actually be false.

The only place where the code writes a null value into this
map is at the end of computeRequestReassignmentForNetwork :
reassignedRequests.put(nri, null).
This proves the code the if() block, which proves that
newNetwork.isSatisfyingRequest(nri.request.requestId) is true.

By definition newNetwork.isSatisfyingRequest(nri) implies that
nri.mSatifier == newNetwork, which proves that
previousSatisfier == newNetwork whenever newSatisfier is null.

Fixes: 146482072
Test: FrameworksNetTests
Change-Id: Ifd6faedce7d49757b82a5f341076ab208b0ccfcb