Adding HIDL interface type to safeunion test HAL

This change adds a new HIDL safe union, InterfaceTypeSafeUnion
(containing an interface+array) to the safeunion test HAL.

Bug: 79878527
Bug: 110269925
Test: Existing hidl_test suite passes successfully. Added more test-
cases to hidl_test_client.cpp to exercise InterfaceTypeSafeUnion
(included in a separate CL).

Change-Id: I04ffeeeabccc1dc2651aac9371569c36d37f4f87