Add key layouts for SteelSeries Stratus+

The Stratus+ is somehow bugged when it uses the generic gamepad file; to
fix this, a Stratus+-specific key layout file is added, similar to the
one for the Stratus Duo.

Bug: b/232827632
Test: Stratus+ USB connection, playing SF4 on eve (ARC-R)
Change-Id: Ied0e27a57b159fd35915634bd62ffced58ced4f2