Create basic data store to store keyboard classification

Implements a basic file system backed data store (using a json
file), to store keyboard classification information persisted
across reboots.

Test: atest --host libinput_rust_test
Bug: 263559234
Flag: com.android.input.flags.enable_keyboard_classifier

Change-Id: I521444342ae4e98191262273b881775fb2157ef2