wifi(interface): Make methods synchronous

Having all the HIDL methods asynchronous, makes it hard for the
calling code to keep track of the operation status/result.
There are some operations which will generate asynchronous results (like
bgscan), convert all the other methods to synchronous methods.

The |EventCallback| objects will now just broadcast important events
(needed for other clients to listen for state changes). This will no
longer be used to send responses to every command sent to the HAL.

Bug: 32061909
Test: `./hardware/interfaces/update-makefiles.sh`
Change-Id: Id2433f4c8e028268dd027cdeb239ba4082b157b5