Use TestableResultReceiver in EntitlementManagerTest

This is a test only refactoring which updates the unit tests to use a
TestableResultReceiver to have better readability, and asserting
no entitlement result received.

Test: atest TetheringTests:EntitlementManagerTest
Bug: 376792293
Change-Id: I5cc02c8ab85ae614e714055c7f8f4c076341f48f
1 file changed