Add flag to mark key usage code as a fallback mapping

We don't yet have a way to determine if a device can actually report a
usage code. Adding a flag to mark usage code as fallback only mapping.

Bug: 297094448
Bug: 317838112
Test: atest inputflinger_tests
Change-Id: I137113d0dc9a1c8abf523b96942486be176d0240
Merged-In: I137113d0dc9a1c8abf523b96942486be176d0240