Update android/input.h docs

Fix doc generation error

Bug: 183013620
Test: ./_scripts/update_api_doc.py ~/android/master/ 2>&1 >/dev/null | grep -i input

Before:
android/input.h:680: warning: unable to resolve link to `RELATIVE_X' for \link command
After:
the line above is not present

Change-Id: I995ac91d8bfd53309607e3cdac305adb68d40d78