Return events from EventHub

Rather than filling some buffer, EventHub will now return events
explicitly. This will simplify the interface and avoid storing the
buffer when no events are being actively produced.

Bug: 211379801
Test: atest libinput_tests inputflinger_tests
Change-Id: Ic4ba7789f899b2de9fbe5a0abe54475b74eb5f82
8 files changed