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
Test: atest inputflinger_tests
Change-Id: I137113d0dc9a1c8abf523b96942486be176d0240
diff --git a/libs/input/InputEventLabels.cpp b/libs/input/InputEventLabels.cpp
index c218e1e..0e627e5 100644
--- a/libs/input/InputEventLabels.cpp
+++ b/libs/input/InputEventLabels.cpp
@@ -430,7 +430,8 @@
     DEFINE_FLAG(VIRTUAL), \
     DEFINE_FLAG(FUNCTION), \
     DEFINE_FLAG(GESTURE), \
-    DEFINE_FLAG(WAKE)
+    DEFINE_FLAG(WAKE), \
+    DEFINE_FLAG(FALLBACK_USAGE_MAPPING)
 
 // clang-format on