Mark stylus buttons mapppings as fallback

We don't have a way to determine if devices can actually report HID
usage keys. Marking them as fallback only.

Bug: 297094448
Bug: 317838112
Test: atest inputflinger_tests
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:45f626a154f66f4cceecaf6e32392741a00529a3)
Merged-In: I45710f9e6237c86613717fab779dbd7cc5c66c86
Change-Id: I45710f9e6237c86613717fab779dbd7cc5c66c86