Implement KeyboardClassifier interface in rust

DD: go/project-imposter-android
This CL includes:
- Rust interface setup

Next CL to include:
- Basic categorization into alphabetic and non-alphabetic
- Updating categorization based on key presses

Test: atest --host libinput_rust_test
Test: atest inputflinger_tests
Bug: 263559234
Flag: com.android.input.flags.enable_keyboard_classifier_rust_impl
Change-Id: I52773be992ddd8efaa9546e0af8b0a78515d931c
diff --git a/libs/input/input_flags.aconfig b/libs/input/input_flags.aconfig
index 560166c..a2192cb 100644
--- a/libs/input/input_flags.aconfig
+++ b/libs/input/input_flags.aconfig
@@ -150,3 +150,10 @@
   description: "Hide touch and pointer indicators if a secure window is present on display"
   bug: "325252005"
 }
+
+flag {
+  name: "enable_keyboard_classifier"
+  namespace: "input"
+  description: "Keyboard classifier that classifies all keyboards into alphabetic or non-alphabetic"
+  bug: "263559234"
+}