wifi(interface): Fix compilation failure

Changes in the CL:
a) Unions are not allowed in the HIDL generated code that is going to be
used in java.
b) Few RTT constants were defined twice in the legacy rtt.h. One set
as enums, the other set as #define's. Remove the duplicated values in
the HIDL interface.

Bug: 31991076
Bug: 31991232
Test: mmm -j32 hardware/interfaces/wifi/1.0/
Change-Id: I03db70c7b89c0be53a7754ab8b34a19ad2c6e536